Chapter 15: Modes of Disease Transmission
In the late summer of 1854, a crowded district of London called Soho was struck by cholera. Within about ten days, several hundred people living within a few streets of one another were dead. Nobody at that time knew that germs caused disease. Most doctors believed that illness rose out of foul-smelling air.
A physician named John Snow doubted that explanation. He walked the district and did something simple. On a street map he marked, house by house, where each death had occurred. The marks did not spread evenly through the bad air. They gathered tightly around one thing — a public hand pump on Broad Street from which families drew their drinking water.
Snow looked for cases that did not fit, because a good explanation must survive them. A workhouse with its own well inside the building lost very few people. Workers at a nearby brewery, who drank what the brewery gave them rather than pump water, were largely spared.
Snow persuaded the parish officials to take the handle off the pump so that nobody could use it. The outbreak was already fading by then, but the map had made a point that could not be unmade. The disease had not floated in on the air. It had travelled in the water, and a route that can be traced is a route that can be cut.
Diagram
Fig. 15.0-1 — Mapping An Outbreak: Broad Street, 1854 Simple line-drawn street plan of a small city district; show a grid of named streets with small blocks representing houses, a short bar beside each affected house to mark deaths, the bars clustering densely around a single marked hand pump at the centre; mark two other pumps at the outer edges with almost no bars near them; label the central pump, the outlying pumps and the streets. Line drawing only, no photograph.
15.1 Health, Hygiene And Personal Cleanliness
Most of the diseases in this chapter reach a person by an ordinary route — a hand, a plate of food, a glass of water, an insect. Before we follow those routes, it is worth looking at the habits that block them. Almost every one of them is a small daily act.
15.1.1 What Health And Hygiene Mean
We often say a person is healthy when they are not ill. That is too narrow. Since 1948, the World Health Organization has defined health as a state of complete physical, mental and social well-being, and not merely the absence of disease. A person may have no infection at all and still be far from healthy.
The practice of keeping that state is called hygiene. It has two halves, and both are needed. Looking after your own body is personal hygiene; looking after the surroundings you share with others is social hygiene, which the next section takes up.
Personal hygiene rests on four things: cleanliness, physical exercise, rest and sleep, and healthy habits. Each of the four is dealt with below.
15.1.2 Keeping The Body Clean
Cleanliness is the first of the four, and the one practised most often. Its purpose is not appearance. It is to remove germs — the everyday word for the microbes, such as bacteria, viruses and protozoans, that cause disease — before they reach a part of the body where they can multiply.
Hands And Skin
Think about what your hands touch in a single day: furniture, books, coins and currency notes, seats and hand-rails in a bus, door handles, pets, tools in a workshop. Any of these may carry germs left there by someone else.
Nothing happens while those germs sit on your fingers. The trouble begins when the fingers go to the mouth, or handle food that goes to the mouth. The route is short, and it is set out in FC 15.1.
Flowchart
FC 15.1 — How Germs Travel From A Surface To The Body Germs on a touched surface → picked up on the fingers → transferred to food, or directly to the mouth, nose or eyes → swallowed or inhaled → multiply inside the body → disease
Every link in that chain can be broken, but the easiest one to break is the second. Washing the hands with soap lifts germs off the skin and rinses them away; plain water alone does not do this nearly as well. Hands must be washed after using the toilet and before handling food. Where water is not available, a hand sanitiser will disinfect them.
The steps that matter in a proper wash are shown in Fig. 15.1-1. Most people scrub the palms and forget the backs of the hands, the gaps between the fingers, and the thumbs — which is exactly where germs stay.
Diagram
Fig. 15.1-1 — Washing The Hands With Soap Sequence of six line drawings of a pair of hands, arranged left to right with arrows between them; show (1) wetting the hands and applying soap, (2) rubbing palm against palm, (3) rubbing each palm over the back of the other hand with the fingers interlaced, (4) rubbing between the fingers, (5) rubbing the thumbs and the fingertips against the opposite palm, (6) rinsing and drying on a clean towel. Label each step with its action. Line drawing, no photograph.
A daily bath keeps the whole skin clean. It washes away germs and the stale sweat that gives out body odour, and it clears the sweat pores that dust and oil tend to block. Pay particular attention to the folds and creases of the skin — between the toes, under the arms, behind the ears — because these stay warm and damp, and germs and fungi grow best there. Undergarments and handkerchiefs sit against the skin all day and should be washed daily.
Hair, Teeth And Mouth
Hair should be washed often and combed regularly. This keeps the scalp clean and free of parasites such as head lice, which spread easily through shared combs, caps and pillows.
Teeth should be brushed at least twice a day — after getting up in the morning and again before going to bed at night. The night brushing matters most, because the flow of saliva falls during sleep and food left on the teeth is not washed away.
Here is the reason behind the rule. Bacteria living in the sticky film on the teeth feed on sugar from sweets, chocolates and other food, and give out acid as they do so. The acid slowly dissolves the hard enamel of the tooth, and the cavity that forms is called dental caries. The same bacteria irritate the gums, which then swell and bleed. Rinsing the mouth after every meal removes much of the food they would otherwise feed on.
Nose, Eyes And Ears
Always breathe through the nose and not through the mouth. The nose is built to clean the air; the mouth is not.
When you sneeze, hold a handkerchief in front of the nose and mouth, and blow the nose into a handkerchief rather than into the open air. A sneeze throws out a fine spray of droplets, and anyone standing close breathes them in.
The eyes should be washed with clean water two or three times a day. Two common eye infections, trachoma and conjunctivitis, are picked up from dust and from contaminated fingers, towels and flies. In conjunctivitis, the thin membrane covering the white of the eye becomes inflamed, so the eye turns red, waters and feels gritty.
Because these germs travel on anything that touches the eye, towels should never be shared, not even within a family. The same holds for any stick or applicator used to apply something to the eye. A shared applicator carries infection straight from one person’s eye into another’s.
The ears need very little attention. Wipe the outer ear with a soft, damp cloth. Never push a pin, a sharp object or a cotton bud into the ear canal: the eardrum lies just inside, it is very thin, and it tears easily. Ear wax is not dirt — it traps dust and slowly works its way outward on its own.
15.1.3 Physical Exercise, Rest And Sleep
Some physical exercise is necessary at every age, and especially for children, adolescents and young people, whose bodies are still growing. For older people, long gentle walks are enough. What matters is that exercise is systematic and regular rather than occasional and severe.
Exercise improves blood circulation. The heart beats more strongly, so more blood, and with it more oxygen and food, reaches the muscles and every other organ. Muscles that are used stay firm; muscles that are never used waste away.
Every organ of the body, including the brain, needs rest, and sleep is how the body takes it. The amount needed changes with age. Very young infants sleep for most of the day, students of your age need roughly eight to ten hours, and adults generally need about seven to nine hours of continuous, undisturbed sleep.
15.1.4 Healthy Habits And A Well-Ventilated Home
A healthy habit is simply a good act repeated at a fixed time until it no longer needs deciding. Going to bed at the same hour each night and taking meals at regular hours are the two that shape all the others.
Going to bed very late, or lying down immediately after dinner, are both poor habits. The bowels should be cleared every day, preferably in the morning.
Three things are best avoided altogether. Tobacco, in any form, damages the lungs and the mouth and is a known cause of cancer. Alcoholic drinks harm the liver, the brain and the nervous system, and the harm reaches past the person into the family around them. Stimulants and sedatives — substances taken to force the body awake or to force it to sleep — disturb the body’s own rhythm of activity and rest when taken without medical advice, and the habit is hard to break once it forms.
Living rooms should be well ventilated. Open windows let fresh air in, which replaces the air the people in the room have already breathed. They also let in sunlight, and the ultraviolet radiation in direct sunlight kills many germs on the surfaces it falls upon. A dark, closed, crowded room is where germs survive longest.
15.3 Vectors And The Housefly
15.3.1 What A Vector Is
Some diseases do not travel on their own. The germ that causes them cannot walk from one person to the next, so it is carried — and the carrier is very often an animal going about its ordinary business.
Such a carrier is called a vector. The word covers any agent that acts as a go-between, picking up a pathogen — a germ capable of causing disease — from one place and delivering it to another.
Many insects and other animals act as vectors, and steps must be taken to control them, or to eradicate the diseases they carry. Three of them do more damage than all the rest and are given the highest priority — the housefly, the mosquito and the cockroach. They are often described as the worst public enemies, and each is taken up in turn in this chapter.
15.3.2 How The Housefly Spreads Disease
The housefly, Musca domestica, is by habit a scavenger. It feeds on leftovers, refuse and waste, and in doing so it clears away material that would otherwise rot where it lies.
The trouble is where it feeds. A fly moves freely between a rubbish heap, an open drain, exposed excreta and the food on your plate, and it takes something of each place with it. The danger it poses to human life is so serious that it is often designated Public Enemy No. 1. Among the diseases it spreads are dysentery, cholera and typhoid.
The housefly does not bite and cannot pierce the skin. Everything it does to us, it does through food. Its external features, shown in Fig. 15.3-1, explain how.
Diagram
Fig. 15.3-1 — External Features Of The Housefly Enlarged side view of a housefly as a line drawing; label the compound eye, antenna, sponging mouthparts (proboscis), thorax, one pair of wings, the halteres, the hairy or spiny legs, the sticky pad at the end of each foot, and the hairy body surface.
There are four separate ways in which it carries germs to human food.
Its hairy body and legs pick up filth. The body, and especially the legs, are covered with hairs and spines, and each foot ends in a sticky pad. Filth containing germs clings to these surfaces at every dirty place the fly visits. When such a fly sits on food, it contaminates it. A resting housefly constantly rubs its legs against one another and over its body, and particles of filth drop off exactly where it is standing.
It pours out saliva on the food. A housefly cannot chew. Its sponging mouthparts can take up only liquids, so it must first moisten solid food by pouring out saliva on it and then suck up the softened material. It may also vomit up — regurgitate — food it swallowed earlier at a dirty place. Both the saliva and the regurgitated matter carry germs from wherever the fly last fed.
It deposits its excreta while feeding. A fly feeds and passes out waste at the same time, leaving small dark specks on the food it has settled on.
It transmits germs directly. Sometimes the fly conveys an infection straight from one person to another without any food in between. A fly that has settled on the infected eyes of a person with trachoma may next settle on the eyes of a healthy person, carrying the germs across on its body.
15.3.3 Control Of The Housefly
To control any vector you must know where it breeds, because that is where it is easiest to kill in large numbers. The housefly lays its eggs in moist decaying matter — refuse, cowdung and horsedung — and passes through the stages shown in FC 15.2.
Flowchart
FC 15.2 — Life Cycle Of The Housefly Eggs laid on refuse or dung → larva (maggot), feeding in the moist waste → pupa, within a hardened case → adult fly (imago) → the adult lays eggs on fresh refuse, returning the cycle to the start
Notice that three of the four stages are spent in one place and cannot move away. That is the weak point, and the first method of control attacks it.
Elimination of breeding places. All refuse from houses, and all animal and human excreta, must be removed at regular and frequent intervals and disposed of so that flies cannot breed in it. Where refuse is covered with a layer of earth, the decomposition going on inside produces enough heat to kill any maggots present.
Spraying. Houses and breeding places are sprayed with insecticides. Sprays reach the adult flies, which the first method cannot.
Avoidance. Food should be kept well protected and covered, so that a fly which survives everything else still cannot reach it. Flies should not be allowed to settle on the body, and sleeping babies should be protected by nets.
The three methods are used together, not one instead of another. Spraying kills the flies that exist today; clearing the breeding places decides how many exist next week; covering the food protects you from those that remain.
15.4 Mosquitoes And The Diseases They Carry
15.4.1 Three Mosquitoes And Their Diseases
Not every mosquito carries every disease. Three genera matter most, and each is tied to particular diseases caused by quite different kinds of germ.
Anopheles spreads malaria in human beings, and also in monkeys, birds and other animals. The malaria germ is a protozoan, a single-celled animal-like organism, named Plasmodium.
Culex spreads filariasis, caused by a nematode worm, Wuchereria. The worms block the lymph vessels of the limbs, and the swelling that follows over the years is called elephantiasis.
Aedes spreads yellow fever and dengue, each caused by its own virus.
These are set out together in Table 15.1.
Notice what the table really shows. One vector can carry a protozoan, another a worm, another a virus. The mosquito supplies the transport; it does not decide the kind of germ.
15.4.2 How A Mosquito Passes Germs From One Person To Another
Only the female mosquito sucks blood. She needs the protein in it to develop her eggs; the male feeds on plant juices and never bites at all.
The germs of these diseases are spread when a mosquito bites an infected person to suck blood. Along with the blood, it takes in the germs of that particular disease. Inside the mosquito’s body the germs grow and develop, and in the case of malaria they also multiply, so that after some days the insect is loaded with them.
When such an infected mosquito next bites a healthy person, the germs are introduced into that person’s body through the mosquito’s saliva. This is worth pausing over. Before sucking blood, a mosquito first injects saliva into the skin. The saliva contains a substance that prevents the blood from clotting, so that it keeps flowing freely up the narrow proboscis instead of sealing itself off. The germs travel in on that saliva.
Diagram
Fig. 15.4-1 — A Female Mosquito Feeding Enlarged side view of a female mosquito resting on a patch of skin shown in section; label the proboscis inserted through the skin, the antenna, the compound eye, the thorax, the wings, the abdomen, a small blood capillary below the skin surface, and arrows marking saliva passing down into the skin and blood being drawn up.
15.4.3 Control Of Mosquitoes
A mosquito spends the first three stages of its life in water and only the last in the air. FC 15.3 shows the sequence, and every method of control is aimed at one stage or another of it. The stages differ slightly between Culex and Anopheles, but the order is the same in both.
Flowchart
FC 15.3 — Life Cycle Of The Mosquito Eggs laid on the surface of still water → larva, hanging below the surface and breathing air through a tube → pupa, also breathing at the surface → adult mosquito emerging into the air → the female feeds on blood and lays eggs on still water, returning the cycle to the start
The larva and the pupa live in water but breathe air, coming up to the surface for it. That single fact is what makes the third method below work, and Fig. 15.4-2 shows why.
Diagram
Fig. 15.4-2 — Mosquito Larva And Pupa At The Water Surface Sectional line drawing of the top few centimetres of a pond; show a larva hanging head-downward with its breathing tube piercing the water surface and a comma-shaped pupa with its breathing trumpets at the surface; label the larva, the breathing tube (siphon), the pupa, the breathing trumpets and the water surface. Beside it draw the same view with a thin oil film over the surface, labelled, and the larva unable to reach air.
Several steps are recommended, aimed at mosquitoes in different stages:
Spraying. Adult mosquitoes are destroyed by spraying insecticides in dwelling places. Spraying on ponds and marshy places kills mosquitoes at the larval stages.
Elimination of breeding places. Breeding places should be destroyed. Small ponds and puddles are filled up, so that there is no still water in which eggs can be laid.
Spraying on stagnant water. Kerosene or some other greasy oil is poured over the water to make a thin film on the surface. The oil kills the larvae and the pupae when they come up to breathe, because they can no longer reach air through it.
Biological control. This is the use of one living organism to keep down the numbers of another. Certain fishes, such as Gambusia, are introduced into ponds, where they selectively devour mosquito larvae.
The four methods together cover the whole life cycle. Filling a puddle prevents eggs; oil and larvicidal spraying deal with the larvae and pupae; fish eat the larvae continuously without anyone returning to the pond; spraying indoors removes the adults that have already emerged.
15.5 Cockroaches And Rats
Flies and mosquitoes fly in from outside. The two pests in this section live in the building itself, which is what makes them difficult to be rid of.
15.5.1 Cockroaches
Cockroaches are very common household insects. They breed mostly in the connecting manholes and the sewers below a building, and from there they extend their activities into kitchens, cupboards and wardrobes.
That journey is the whole problem. An insect that has been walking in a sewer walks a few hours later across the shelf where flour and bread are kept. Cockroaches eat and spoil food, and they also damage paper and clothes.
Cockroaches carry germs on their legs and body and in their droppings, in much the same mechanical way as the housefly, and so contaminate whatever food and utensils they walk over. Their droppings and cast-off skins are also a well-recognised trigger of asthma and other allergies in people who live with a heavy infestation.
The features to look for are shown in Fig. 15.5-1.
Diagram
Fig. 15.5-1 — External Features Of A Cockroach Dorsal view of a cockroach as a line drawing, with the wings of one side drawn spread to show the segments beneath; label the long antennae, compound eye, head, pronotum covering the thorax, the two pairs of wings, the three pairs of spiny legs, the segmented abdomen, and the pair of cerci at the hind end.
Spraying insecticides in their breeding places is an effective method of control. The word ‘breeding places’ is important — spraying a kitchen shelf kills the cockroaches on it, while treating the manhole and drain reaches the population that keeps replacing them.
15.5.2 Rats And The Rat Flea
Rats are serious domestic pests. Besides eating grains and other stored food, they spread the germs of certain diseases.
The most serious of these is not carried by the rat directly. A rat flea is a small wingless insect that lives as a parasite on the rat’s skin and feeds on its blood, and it is the flea that may carry the germs of plague, a bacterial disease caused by Yersinia pestis. FC 15.4 shows how the germ reaches a person.
Flowchart
FC 15.4 — How Plague Reaches A Person Bacteria multiply in the blood of an infected rat → a rat flea feeds on that rat and takes in the bacteria → the infected rat dies → the flea leaves the dead body in search of a new host → it bites a person nearby → bacteria enter that person’s blood and plague develops
Read the fourth step again, because it explains something that once puzzled people badly. A sudden death of rats in a locality was followed, not prevented, by human cases — the fleas had lost their usual host and moved to the next warm body available.
Plague spread as a serious epidemic in many parts of the world during the earlier part of the twentieth century, killing a great number of people. India was among the countries severely affected.
Rats should therefore be trapped and disposed of, or killed with suitable rat poisons, which are called raticides. Storing grain in closed metal or hard plastic containers removes the food supply that keeps a rat population going in the first place.
15.6 Contamination Of Water
15.6.1 What Contamination Means
Water that looks clear may still be unfit to drink. Germs are far too small to see, and they leave no colour, no smell and no taste behind them.
Contamination is the word for what has happened to such water.
Water fit to drink is called potable water, and a supply of clean, germ-free potable water is essential to personal health and to public health alike. It is also the single point at which a whole community stands or falls together. One contaminated well can make an entire village ill within days, while one careless person cannot.
15.6.2 Principal Sources Of Contamination
Contamination of potable water occurs in several ways.
- Sewage, if improperly disposed of, leaks into the subsoil and may contaminate underground water supplies. This is the least visible route of all: the leak is below ground, and the well or handpump that draws from the same layer of soil gives water that looks perfectly clean.
- People defecating or urinating near lakes, rivers or ponds add germs directly to water that others will drink, wash in, or draw for cooking.
- Animal wastes, and the washings from dairies and poultry farms, may be drained into water bodies.
What these three have in common is worth naming. In every case, material from the gut of a human being or an animal reaches drinking water. The germs of the diseases dealt with in the next section leave the body in exactly that material, which is why safe disposal of excreta and safe water supply are really one problem and not two. Fig. 15.6-1 shows the commonest path.
Diagram
Fig. 15.6-1 — How Sewage Reaches A Well Through The Subsoil Sectional line drawing of the ground: on the left a leaking soak pit or open drain, on the right a well or handpump about fifteen metres away, both cut open to show depth; label the soil layers, the water table, the leaking pit, arrows showing seepage of sewage sideways through the subsoil into the groundwater, the well drawing from that same groundwater, and the bucket or tap at the surface.
15.6.3 Industrial And Agricultural Wastes In Water
Besides these principal sources, a variety of industrial and agricultural wastes reach water bodies. These may contain poisonous chemicals, which are harmful to human health once they mix with water people use.
- Industrial wastes containing mercury cause abnormalities in the nervous system. Mercury is not broken down and not easily thrown out of the body, so it builds up in the tissues of fish and then in the people who eat them.
- Tannery wastes from leather industries are serious water pollutants. Hides may carry the spores of the bacterium responsible for anthrax, and these can pass into the effluent.
- Pesticides, especially older ones such as DDT, are serious water pollutants. They dissolve in fat rather than water, so they collect in the bodies of organisms and become more concentrated at each step of a food chain. Some interfere with the action of hormones, and long-term exposure to certain pesticides has been linked to a raised risk of cancer.
- Fluorides in drinking water cause tooth disorders and bone disorders, and at higher concentrations they cause neuro-muscular disorders and may be fatal.
15.7 The Three Common Water-Borne Diseases
If the agencies supplying water are not careful about purifying it, the residents of the area may suffer from three common diseases — cholera, dysentery and hepatitis. Each of them may take the form of a severe epidemic, meaning that a large number of people in one area fall ill with the same disease at about the same time.
A water supply is shared, and that is what makes an epidemic possible. A germ that reaches the supply reaches every household drawing from it, on the same day, in the same glass.
The three diseases attack different parts of the digestive system, as Fig. 15.7-1 shows, and each is caused by a different kind of germ.
Diagram
Fig. 15.7-1 — Where The Three Water-Borne Diseases Act Simplified outline of the human digestive system as a line drawing; label mouth, oesophagus, stomach, small intestine, large intestine, liver and gall bladder. Mark the small intestine as the site attacked in cholera, the large intestine as the site attacked in dysentery, and the liver as the organ inflamed in hepatitis, with a leader line and label for each.
15.7.1 Cholera
Cholera is an infection of the intestinal tract, caused by a bacterium, Vibrio cholerae.
The bacterium does not eat away the intestine. It releases a poison that forces the lining of the intestine to pour out water and salts instead of absorbing them. Everything that follows comes from that one change.
The symptoms are severe diarrhoea and vomiting, followed by dehydration — the loss of far more water from the body than is being taken in. As the water in the body runs short, urination stops. Urea, a waste substance that the kidneys normally remove in urine, then accumulates in the blood, where it is highly poisonous, and death may follow.
Infection occurs through contaminated water. It also occurs through dust, and through flies that settle on excreta and transfer the germs to exposed food. Food handlers with unwashed hands can transmit the germs in the same way.
15.7.2 Dysentery
Dysentery is an infection of the large intestine marked by loose motions. It takes two forms, and the difference between them is not in the symptoms but in the germ responsible.
15.7.2.1 Bacillary Dysentery
The causative agent is a bacterium, Shigella. It occurs frequently among children.
The disease is marked by loose motions, that is diarrhoea, together with intestinal pain and mild fever. People with the infection are treated with antibiotics, medicines that act against bacteria.
Prevention rests on two habits: drinking safe water, boiled if there is any doubt about it, and eating food that has been kept covered and out of the reach of flies.
15.7.2.2 Amoebic Dysentery
The causative agent here is not a bacterium but a protozoan, Entamoeba histolytica, shown in Fig. 15.7-2.
Diagram
Fig. 15.7-2 — Entamoeba histolytica Enlarged drawing of a single amoeba with an irregular outline; label the single large blunt pseudopodium, the nucleus, the granular cytoplasm, and two or three ingested red blood cells within it. Note in the caption that no contractile vacuole is present.
This organism enters the body through contaminated water and food. It establishes itself in the lining of the large intestine, where it feeds by engulfing the red blood cells of the intestine of the person infected. It has no contractile vacuole, and it bears one large blunt pseudopodium.
When fully mature, the organism forms cysts, resting stages enclosed in a tough wall, which pass out of the body through the faeces. These cysts remain active for a long time outside the body — which is exactly why the disease travels so well in water. On reaching a new host through contaminated water or food, they split open to infect a new individual. FC 15.5 shows the whole round.
Flowchart
FC 15.5 — Life Cycle Of Entamoeba histolytica Cysts in contaminated water or food → swallowed → cysts split open in the intestine and release amoebae → amoebae settle in the lining of the large intestine and feed on red blood cells → mature amoebae form new cysts → cysts pass out in the faeces → cysts survive a long time and reach water or food again, returning the cycle to the start
The two forms of dysentery are compared in Table 15.2.
15.7.3 Hepatitis
The name hepatitis comes from hepatos, the Greek word for the liver, and means inflammation of the liver.
The types usually named are three. Hepatitis A is the infectious hepatitis, and it is the one transmitted through contaminated water. The other two, hepatitis B and hepatitis C, are passed through blood and other body fluids, and so are not water-borne at all.
The symptoms follow from the liver being unable to do its ordinary work:
- body ache;
- yellowish eyes and deep yellow urine, caused by bile pigments building up in the blood instead of being handled by the liver;
- an enlarged liver.
Prevention rests on three measures. Vaccines are available against hepatitis A and hepatitis B and give lasting protection. Proper hygiene — safe drinking water, covered food, washed hands — blocks the route by which hepatitis A travels. While the liver is recovering, a light diet is usually advised, avoiding fat-rich foods, since fatty food adds to the work the liver has to do.
15.8 Summary
Health is the condition of a person who is fit in body, mind, emotions and social life, and not merely free from disease.
Hygiene is the science and practice of maintaining good health. Personal hygiene is the care of one’s own body; social hygiene is the care of shared surroundings.
The four major aspects of personal hygiene are cleanliness, physical exercise, rest and sleep, and healthy habits.
Hands pick up germs from everything they touch and carry them to the mouth, so washing with soap, especially after using the toilet, is essential. A daily bath keeps the skin clean and free of the bacteria that produce body odour, with attention to the folds of the skin.
Hair must be washed and combed to keep it free of parasites such as head lice. Undergarments and handkerchiefs must be washed daily.
Teeth should be cleaned twice a day and the mouth rinsed after meals. Excess sweets and chocolate cause dental caries and gum infections.
One should breathe through the nose, blow the nose into a handkerchief, and cover the nose and mouth while sneezing.
Eyes should be washed with clear water daily. Trachoma and conjunctivitis spread through dust, hands, shared towels and shared kajal sticks.
Ears should be cleaned on the outside only, and no sharp object should ever be put into the ear, as the eardrum is easily injured.
Physical exercise must be systematic and regular; it improves blood circulation. All organs, including the brain, need rest, which the body obtains chiefly through sleep.
Healthy habits include fixed hours for meals and sleep and daily clearing of the bowels. Tobacco, stimulants, sedatives and alcoholic beverages are harmful and should be avoided. Living rooms should be well ventilated for fresh air and sunlight.
Sanitation covers clean public places, clean eating places with covered food and properly washed utensils, covered drains and garbage sprinkled with disinfectants such as bleaching powder and lime, and civic bodies supplying germ-free water and disposing of sewage.
A vector is any agent that acts as an intermediate carrier of a pathogen. The housefly, the mosquito and the cockroach are given the highest priority in control.
The housefly is a scavenger that spreads dysentery, cholera and typhoid in four ways — filth on its hairy body and legs, saliva and regurgitated food poured on our food, its excreta, and direct transfer of germs from one person to another.
The housefly breeds in refuse and dung, passing through egg, larva (maggot), pupa and adult stages. It is controlled by eliminating breeding places, by spraying insecticides, and by keeping food covered.
Anopheles spreads malaria, caused by the protozoan Plasmodium; Culex spreads filariasis, caused by the nematode worm Wuchereria; Aedes spreads yellow fever and dengue, caused by viruses.
A mosquito takes in germs while sucking blood from an infected person. The germs develop inside it, and are injected into a healthy person with the saliva, which contains a substance that prevents blood from clotting.
Mosquitoes breed in still water and are controlled by spraying insecticides, filling up ponds and puddles, spreading a film of oil on stagnant water so that larvae and pupae cannot breathe, and biological control using fishes such as Gambusia.
Cockroaches breed in manholes and sewers, enter kitchens and cupboards, spoil food, paper and clothes, and are controlled by spraying insecticides in their breeding places.
Rats eat and spoil stored grain. The rat flea, a parasite on the rat’s skin, may carry the germs of plague, which spread as a serious epidemic in the earlier part of the twentieth century. Rats are trapped or killed with raticides.
Contamination is the unwanted entry of disease-causing germs into drinking water or edible food. Potable water is contaminated chiefly by leaking sewage, by defecation and urination near water bodies, and by animal wastes from dairies and poultry farms.
Industrial and agricultural wastes add poisons to water: mercury causes abnormalities of the nervous system, tannery wastes may carry the germ of anthrax, pesticides accumulate in the body, and excess fluoride causes tooth, bone and neuro-muscular disorders.
Three common water-borne diseases are cholera, dysentery and hepatitis, and each may take the form of an epidemic.
Cholera is caused by the bacterium Vibrio cholerae and shows severe diarrhoea and vomiting, followed by dehydration; urination stops and urea accumulates in the blood.
Bacillary dysentery is caused by the bacterium Shigella and is marked by loose motions, intestinal pain and mild fever. Amoebic dysentery is caused by the protozoan Entamoeba histolytica, which lives in the lining of the large intestine, engulfs red blood cells, and forms long-lasting cysts that pass out in the faeces and infect a new host.
Hepatitis means inflammation of the liver. Hepatitis A is transmitted through contaminated water, while hepatitis B and C pass through blood and other body fluids. Its signs are body ache, yellowish eyes, deep yellow urine and an enlarged liver.
Flowchart
FC 15.6 — The Chapter In One Chain Germs leave an infected person or animal in excreta, discharges or blood → they reach refuse, soil, still water or a water supply → they are carried onward by water, by contaminated food, or by a vector such as a housefly, mosquito, cockroach or rat flea → they enter a healthy person through the mouth or through the skin → disease develops, and may spread as an epidemic → the chain is broken by personal hygiene, sanitation, vector control and safe drinking water
15.9 Key Terms
- Biological control — keeping down the numbers of a pest by using another living organism, such as Gambusia fish that eat mosquito larvae.
- Cholera — an infection of the intestinal tract caused by the bacterium Vibrio cholerae, marked by severe diarrhoea, vomiting and dehydration.
- Conjunctivitis — inflammation of the conjunctiva, the thin layer covering the white of the eye and lining the eyelid.
- Contamination — the unwanted entry of disease-causing germs into drinking water or edible food.
- Dengue — a viral disease spread by the Aedes mosquito.
- Dental caries — decay of a tooth, in which acid produced by bacteria dissolves the enamel and forms a cavity.
- Dysentery — an infection of the large intestine marked by loose motions, caused either by the bacterium Shigella or by the protozoan Entamoeba histolytica.
- Epidemic — the occurrence of the same disease in a large number of people in one area at about the same time.
- Filariasis — a disease spread by the Culex mosquito and caused by the nematode worm Wuchereria, whose long-term effect is elephantiasis.
- Fluoride — a substance present in some groundwater which strengthens enamel in tiny amounts but causes tooth, bone and neuro-muscular disorders in excess.
- Germs — micro-organisms too small to be seen with the eye, which cause disease.
- Health — the condition of a person who is fit in body, mind, emotions and social life, and not merely free from disease.
- Hepatitis — inflammation of the liver; hepatitis A is carried by contaminated water, hepatitis B and C by blood and body fluids.
- Hygiene — the science and practice of maintaining good health.
- Malaria — a disease spread by the Anopheles mosquito and caused by the protozoan Plasmodium.
- Pathogen — a germ capable of causing disease.
- Personal hygiene — the care a person takes of his or her own body.
- Pesticide — a chemical used to kill pests, which may collect in living bodies and become more concentrated along a food chain.
- Plague — a bacterial disease caused by Yersinia pestis and carried to people by the rat flea.
- Potable water — water that is fit to drink.
- Raticide — a poison used to kill rats.
- Sanitation — the practical work of keeping surroundings free of the wastes and filth in which germs breed.
- Sewage — waste water carrying human excreta and household washings.
- Social hygiene — the care taken of surroundings shared by a whole community.
- Trachoma — a long-lasting infection of the surface of the eye which, on repeated attack, can scar the eyelid and damage sight.
- Vector — any agent that acts as an intermediate carrier of a pathogen from one host to another.
- Yellow fever — a viral disease spread by the Aedes mosquito.
15.10 Points To Ponder
The housefly is described both as a natural scavenger and as Public Enemy No. 1. Can the same habit make an organism useful and dangerous at once? What might happen in a locality if every housefly disappeared overnight?
Four methods are used against mosquitoes, each aimed at a different stage of the life cycle. If a town could afford only one of them, which stage would you attack, and what would you be giving up by ignoring the others?
In cholera the bacterium never leaves the intestine, yet a person may die of it. What does that tell you about how a disease actually causes harm, as against where the germ happens to sit?
The amoeba causing amoebic dysentery cannot survive long outside the body, but its cyst can. Why would a parasite need such a stage at all, and what does the cyst tell you about the journey between one host and the next?
John Snow stopped people drinking from the Broad Street pump years before anyone knew what caused cholera. Must you know the cause of a disease in order to prevent it? Can you think of a reason why knowing the cause is still worth having?
Personal hygiene protects one person and social hygiene protects a whole community. Which of the two collapses more completely without the other, and what does your answer suggest about where public money should go first?
15.11 Objective Questions
Multiple Choice Questions
- The germ that causes malaria is a: (1 mark)
- bacterium
- virus
- protozoan
- nematode worm
- Filariasis, whose long-term effect is elephantiasis, is spread by: (1 mark)
- Anopheles
- Culex
- Aedes
- the housefly
- A thin film of oil spread over stagnant water controls mosquitoes chiefly because it: (1 mark)
- poisons the eggs on the surface
- stops the larvae and pupae from reaching air
- kills the adults resting on the bank
- makes the water unsuitable for egg laying only
- Amoebic dysentery is carried from one person to another chiefly by the: (1 mark)
- adult amoeba in water
- cyst passed out in the faeces
- pseudopodium of the amoeba
- bite of an infected fly
- Which one of the following is a measure of social hygiene rather than personal hygiene? (1 mark)
- Washing the hands with soap after using the toilet
- Combing the hair regularly
- Covering public drains and sprinkling bleaching powder on them
- Cleaning the teeth twice a day
Assertion-Reason
- In each of the following, an assertion (A) is followed by a reason (R). Choose from: (1 mark each)
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): A mosquito injects its saliva into the skin before sucking blood. Reason (R): The saliva contains a substance that prevents the blood from clotting.
- Assertion (A): Covering a refuse heap with a layer of earth helps to control houseflies. Reason (R): The heat produced by decomposition inside the covered refuse kills the maggots.
- Assertion (A): Boiling makes water from any source completely safe to drink. Reason (R): Boiling destroys the germs present in water.
Match The Following
- Match each organism in Column A with the disease in Column B. (4 marks)
| Column A | Column B |
|---|---|
| a. Vibrio cholerae | i. Bacillary dysentery |
| b. Shigella | ii. Plague |
| c. Entamoeba histolytica | iii. Cholera |
| d. Yersinia pestis | iv. Amoebic dysentery |
Fill In The Blanks
- Complete the following: (4 marks)
- Water that is fit to drink is described as …………. water.
- The larval stage of the housefly is commonly called a ………….
- Fishes such as …………. are introduced into ponds to devour mosquito larvae.
- The name hepatitis means inflammation of the ………….
True Or False
- State whether each statement is true or false: (4 marks)
- Both the male and the female mosquito suck blood.
- The housefly spreads disease by biting human beings.
- Hepatitis A is transmitted through contaminated water.
- Bleaching powder acts as a disinfectant because it releases chlorine.
Name The Following
- Name the following: (4 marks)
- The stage of Entamoeba histolytica that survives for a long time outside the body.
- The parasite living on the skin of a rat that may carry the germs of plague.
- The waste substance that accumulates in the blood when urination stops in cholera.
- The insect that breeds mainly in manholes and sewers and later enters kitchens and cupboards.
Odd One Out
- Pick the odd one out in each group and give a reason for your choice: (3 marks)
- Malaria, filariasis, dengue, plague
- Egg, larva, pupa, cyst
- Mercury, fluoride, pesticide, Shigella
15.12 Check Your Concepts
Define a vector, and name the three vectors given the highest priority in control. (2 marks)
Give a reason: cleaning the teeth at night is more important than cleaning them at any other time. (2 marks)
State two ways in which sewage and human waste may contaminate drinking water. (2 marks)
Why should one breathe through the nose and not through the mouth? (2 marks)
Describe three different ways in which a housefly contaminates human food. (3 marks)
In cholera, why does urination stop, and what follows in the blood as a result? (3 marks)
Differentiate between personal hygiene and social hygiene, stating the basis of difference. (3 marks)
Name the four methods used to control mosquitoes, and state for each the stage of the life cycle it attacks. (4 marks)
Differentiate between bacillary dysentery and amoebic dysentery on the basis of the causative agent, the way the gut is damaged, and the stage by which the disease spreads. (4 marks)
What would happen if a town supplied properly chlorinated drinking water but made no arrangement for the disposal of sewage? Explain your answer with reference to two diseases. (4 marks)
Explain, step by step, how the malaria parasite passes from an infected person to a healthy person through the mosquito. (5 marks)
The housefly is called a natural scavenger and also Public Enemy No. 1. Explain both descriptions, and state three measures by which houseflies are controlled. (5 marks)
15.13 Figure-Based Questions
Study Fig. 15.3-1. Name the insect shown, and name any four labelled parts of its body that help it to carry germs from filth to food. State how any one of them does so. (4 marks)
Look at Fig. 15.4-2. Identify the two stages shown, name the structure through which each of them obtains air, and explain how a film of oil on the water surface kills them. (4 marks)
Fig. 15.7-2 shows a single-celled organism. Name it, name the disease it causes, and label the pseudopodium and the ingested red blood cells. State one feature by which it differs from Amoeba found in pond water. (4 marks)
Study Fig. 15.6-1. Name the route by which the germs reach the well, and state two steps a village could take to prevent contamination of this kind. (3 marks)
Redraw the life cycle shown in FC 15.2, naming each of the four stages in the correct order, and mark the stage at which covering refuse with earth acts. (3 marks)
15.2 Social Hygiene And Sanitation
A person can wash their hands ten times a day and still fall ill if the water in the tap is unsafe or the drain outside the door is open. Personal hygiene protects one body. Sanitation — the arranging of a settlement so that human waste, refuse and water never meet — protects everybody in it at once, including those who cannot protect themselves.
15.2.1 Clean Public Places And Clean Eating Places
All public places should be kept clean: schools, office buildings, bathrooms, lavatories and cinema halls. These are exactly the places where large numbers of people touch the same few surfaces in a single day, so germs left by one person can reach many.
Eating places need more care than any other, because there the germs go straight into food. Hotels, restaurants, roadside dhabas and the ground around them must be kept clean and free of flies. All eatables should be kept covered, since an uncovered dish is an open invitation to a fly that has just left a heap of refuse.
Utensils, cups, glass tumblers and spoons must be washed thoroughly, and preferably with soap and hot water. Hot water matters for a reason: it melts and lifts away the film of grease in which germs are held, and the soap then carries that film off the surface. A quick rinse in cold water leaves the film, and the germs, in place. Fig. 15.2-1 sets the two habits side by side.
Diagram
Fig. 15.2-1 — Two Ways Of Running An Eating Place Two line drawings side by side, framed as a pair. Left panel: an uncovered dish of food on a counter with flies above it, an open waste bin beside it, and utensils stacked in a bucket of cold water; label ‘food uncovered’, ‘open waste bin’, ‘utensils rinsed in cold water’. Right panel: the same counter with the dish under a mesh cover, a lidded waste bin, and utensils being washed with soap in hot water; label ‘food covered’, ‘covered waste bin’, ‘utensils washed in soapy hot water’.
On your way home, look at how food is kept at two or three shops or stalls — covered or open, near a drain or away from one. Note which arrangement would keep flies off. Watch only; there is nothing to ask anyone.
15.2.2 Drains, Garbage, Safe Water And Sewage Disposal
Public drains and garbage should be kept covered. An open drain and an uncovered heap of refuse are the two best breeding grounds a settlement can offer to flies and mosquitoes, and both lie within a few metres of the food people eat.
Disinfectants are sprinkled over drains and refuse at frequent intervals. A disinfectant is a chemical that kills germs on a surface rather than inside the body. Bleaching powder and lime are the two used most often, because they are cheap enough to use over a whole locality.
Bleaching powder works by slowly releasing chlorine, which destroys the germs it meets. Lime works differently: it makes a surface strongly alkaline, and very few germs survive in it. Both are for drains, refuse and floors, never for the skin.
Two duties are too large for any household and belong to the civic bodies — the municipalities and corporations. The first is the supply of clean, germ-free drinking water to every home. The second is the efficient disposal of sewage, the waste water carrying human excreta and household waste away from a settlement.
These two duties are really one duty, seen from both ends. Sewage carried away carelessly finds its way back into wells, rivers and pipelines, and the water supply then delivers to every house the very germs the sewage was meant to remove. Section 15.6 follows that return journey in detail.
Chlorinating A Town’s Water Supply Before piped water reaches a town, it is usually filtered to remove mud and grit, and then treated with a small, measured quantity of chlorine, often from bleaching powder. The chlorine kills the bacteria that survive filtration, and a trace of it stays in the water all the way through the pipes, so that germs entering through a leak are dealt with too. This is why treated tap water sometimes carries a faint smell of chlorine, and it is one of the cheapest public-health measures ever devised. A household that is unsure of its supply can achieve the same end by boiling the water.