Chapter 16: Diseases: Cause And Control
In 1876, in the small town of Wollstein in Prussia, a country doctor named Robert Koch was worried about the sheep and cattle around him. Year after year the animals were dying of anthrax, and no one could say why. Some people blamed bad air, some blamed the soil, and some blamed bad luck.
Koch decided to look instead of guess. He set up a microscope in a corner of his own consulting room, took a drop of blood from an animal that had died, and found it crowded with tiny rod-shaped bodies. Blood from healthy animals had none.
That alone proved nothing. Anyone could argue that the rods were a result of the illness rather than its cause. So Koch injected a little of the infected blood into healthy mice. The mice fell ill, died, and their blood carried the same rods.
Then he took the final step. He grew the rods outside any animal, in fluid taken from an ox’s eye, and passed them from one drop of fresh fluid to the next through several generations, until nothing of the original blood was left. He injected this pure growth into healthy animals. They developed anthrax.
Disease, Koch had shown, is not a punishment and not an accident. A definite living cause enters the body, multiplies in it, and can be traced.
Diagram
Fig. 16.0-1 — Koch’s Test Of A Single Cause Line drawing in four panels reading left to right: (1) a dead sheep with a drop of its blood on a slide; (2) the microscope view showing short rod-shaped bacteria among round blood cells; (3) a healthy mouse being injected with the infected blood and shown ill in the next frame; (4) a flask of clear fluid in which the rods are being grown, with an arrow to a healthy animal that then falls ill. Label each panel: blood from dead animal, rods seen under microscope, injection into healthy animal, pure growth of the germ.
16.1 What Is A Disease?
Your body is working at this moment in a hundred quiet ways. Your heart is pumping at a steady rate, your intestine is absorbing food, your kidneys are filtering your blood. As long as every part works in its normal way, you feel well.
A disease is a condition in which this normal working of the body is disturbed. Something in the body’s structure, or in the way that structure functions, has gone off its usual course. You notice it because the body tells you so — through pain, fever, weakness, a rash or a cough.
Not every disease is serious. Many are mild, last a few days and clear up completely, sometimes without any treatment at all. Others are severe, and a few can end a person’s life if they are not treated in time.
Read that definition carefully. It has two halves, and both matter. In some diseases a structure is damaged — a joint wears down, a lung tissue is destroyed. In others the structures look normal but a function fails — a gland makes too little of a substance, or the body cannot use a food properly.
From Sorcery To Science
For most of human history, people had no way of seeing what caused illness. Disease was widely believed to be the work of gods, spirits or an enemy’s curse. The treatments matched the belief: charms, chants, magic and sorcery.
Science replaced that belief with a simple and powerful idea. Every disease is a condition with a definite cause, and that cause can be found. This single change is what makes prevention and cure possible, because you cannot block a cause you have not identified.
To keep this idea before the public, World Health Day is observed on 7 April every year. Its purpose is to make people conscious of staying healthy rather than only of getting treated.
16.2 How Diseases Are Classified
There are thousands of diseases, and no one can study them one by one. So they are sorted into groups. Sorting is not a memory exercise — it is how a doctor decides what to look for and how a government decides what to do.
The same disease can be sorted in more than one way, depending on the question being asked. Ask how far a disease has spread and you get one grouping; ask what causes it and you get another. Four such bases are used in this chapter, and they are set out in FC 16.1.
Flowchart
FC 16.1 — Four Bases For Classifying Diseases A disease → (branch 1) extent of occurrence → endemic / epidemic / pandemic / sporadic; (branch 2) communicability → non-communicable / communicable; (branch 3) kind of pathogen → bacterial / viral / protozoan / helminthic; (branch 4) kind of transmitting agent → air-borne / water-borne / food-borne / insect-borne
Extent of occurrence asks how widely the disease is found — in one small region, across a country, over the whole world, or only in scattered single cases.
Communicability asks whether the disease can pass from one person to another. Diseases caused by germs usually can. Diseases caused by the body’s own poor functioning cannot.
Kind of pathogen asks which type of organism is responsible — a bacterium, a virus, a protozoan, or a worm. This is the question that decides the medicine, because a drug that kills bacteria does nothing to a virus.
Kind of transmitting agent asks how the germ travels — through air, through water, through food, or through an insect. This is the question that decides prevention, because you block a disease at the road it travels by.
Notice that these four bases do not compete with one another. A single disease can be described by all four at once: how far it has spread, whether it is catching, what causes it, and how it travels.
16.3 Diseases Grouped By Extent Of Occurrence
The first question you can ask about a disease is a geographical one. How much ground does it cover, and how many people does it reach at one time?
The answer sorts diseases into four groups. Three of them — endemic, epidemic and pandemic — form a ladder of spreading, and the fourth stands apart from that ladder altogether. Table 16.1 sets the first three side by side.
16.3.1 Endemic Diseases
A disease is called endemic when it is found in one particular area only, and attacks a comparatively small number of people there. It does not disappear, and it does not spread far. It simply stays.
Yellow fever behaves in this way in certain African countries. Goitre, a swelling of the thyroid gland in the neck, behaves in this way in the sub-Himalayan regions of India.
An endemic disease usually stays put because its cause stays put. The soil and drinking water of the sub-Himalayan belt are poor in iodine, and the thyroid gland cannot make its hormone without iodine. The cause is fixed in the ground, so the disease is fixed in that region too.
16.3.2 Epidemic Diseases
A disease is called epidemic when it breaks out suddenly, spreads from place to place, and affects large numbers of people at the same time. The word carries two ideas at once — many people, and a short burst of time.
In 1994, an outbreak of plague began in Surat in Gujarat. As people left the city, cases were reported from several other parts of the country within days. That is the shape of an epidemic: a sudden rise, a spread outwards, and then a fall once control measures take hold.
16.3.3 Pandemic Diseases
A disease is called pandemic when it is distributed widely over the world, crossing countries and continents. A pandemic is an epidemic that has stopped respecting borders.
AIDS and COVID-19 are both examples. Each began in one place, travelled with infected people, and was soon reported from nearly every country. You will study both in detail later in this chapter.
16.3.4 Sporadic Diseases
A disease is called sporadic when it occurs as scattered single cases, here and there, with no regular pattern connecting them. Cases of cholera and of malaria often appear in this scattered way in places where the disease is not otherwise present.
Sporadic does not describe how far a disease has travelled, but how thinly it is scattered. A single case reported this month in one town, and another two months later a hundred kilometres away, is a sporadic occurrence.
16.4 Diseases Grouped By Communicability
The second question you can ask about a disease is the one that matters most to the people around a patient. Can it pass from this person to another?
On this basis, every disease falls into one of two groups. A non-communicable or non-infectious disease cannot pass from one person to another. A communicable or infectious disease can. Table 16.2 places the two side by side.
16.4.1 Non-Communicable Or Non-Infectious Diseases
In these diseases there is no germ of any kind. Nothing is passed from a patient to a healthy person by touch, by air, by shared food or by any other method. They are non-transmissible.
The cause lies inside the body itself. A gland may be working poorly, a nutrient may be missing from the diet, a faulty gene may have been inherited, or a tissue may have worn out with age.
Diabetes, colour blindness, heart attack and beri-beri are all of this kind. You may sit beside a person with any of these conditions for years and never develop it yourself.
16.4.2 Kinds Of Non-Infectious Diseases
Non-infectious diseases are themselves of several kinds, according to what has gone wrong inside the body. FC 16.2 gives a simplified classification with common examples.
Flowchart
FC 16.2 — A Simplified Classification Of Non-Infectious Diseases Non-infectious diseases → (1) nutritional deficiency diseases → beri-beri, scurvy, goitre, kwashiorkor, night-blindness; (2) metabolic diseases → diabetes mellitus, goitre (hypothyroidism); (3) genetic diseases → haemophilia, thalassemia; (4) allergies → hay fever, asthma; (5) degenerative (ageing) diseases → arthritis, cataract; (6) mental illness → depression, schizophrenia; (7) cancer → breast cancer, prostate cancer, leukemia
Nutritional deficiency diseases appear when a nutrient the body cannot make for itself is missing from the food eaten over a long period. Metabolic diseases appear when a chemical process of the body runs at the wrong rate, usually because a hormone is in short supply.
Genetic diseases are inherited from the parents through the genes, and are present from birth. Allergies are over-reactions of the body’s defence system to substances such as pollen or dust, which are harmless to most people.
Degenerative diseases come with the wearing down of tissues over the years. Arthritis is the inflammation of joints that follows deteriorating metabolism in them, though arthritis can also be caused by an infection. Cataract is a clouding of the lens of the eye.
Mental illnesses, such as depression and schizophrenia, are disorders of the working of the brain. They are medical conditions, they can be treated, and a person living with one deserves exactly the care and courtesy given to a person with any other illness.
When Cell Division Goes Out Of Control
Cancer is an abnormal multiplication of cells. Cell division is a normal and necessary activity, but in cancer it no longer stops when it should, and the uncontrolled divisions may build up a mass of cells called a tumour.
For this reason cancers occur most commonly in tissues where cell division is a routine activity — the skin, the liver, the lining of the stomach, the uterus and the breasts. Cancers are often fatal, but early detection and treatment improve the chances of survival a great deal.
Many causes have been suggested. They include chemicals, the tars from tobacco smoking and from tobacco chewing, certain drugs, pollution, certain radiations, and even certain viruses. Any agent that causes cancer is called a carcinogen.
16.4.3 Communicable Or Infectious Diseases
These diseases are caused by germs. The germ somehow reaches a healthy person from an infected person, and the disease appears in the healthy person as well.
A disease-causing germ is called a pathogen. The word is built from two Greek roots — pathos, meaning disease, and gen, meaning producing — so a pathogen is literally a disease-producer. The entry and transmission of such germs from one person to another is called infection.
An infectious disease does not usually appear the moment the infection happens. The germs must first multiply inside the body until they are numerous enough to disturb its working, and this takes time — sometimes a few hours, sometimes many days.
Cholera, malaria and chicken pox are common examples. Smallpox was once among the most feared of them; it no longer occurs anywhere, having been declared eradicated worldwide in 1980 after a global vaccination campaign — the only human disease so far removed from the Earth.
16.4.4 Pathogens And The Diseases They Cause
Pathogens are not one kind of organism. They come from four very different groups, and Table 16.3 lists them with some of the diseases they cause in humans.
This table is the plan of the rest of the chapter. The four groups differ so greatly in size and structure that a treatment which destroys one may leave another untouched, which is why naming the pathogen is the first step in treating any infection.
16.4.5 The Incubation Period
Between the entry of the germ and the first sign of illness there is a silent stretch of time. This is the incubation period, and every infectious disease has one.
During this time the person feels perfectly well, yet the germs are multiplying inside. This is what makes infectious disease so hard to contain: a person can pass on a germ before knowing that anything is wrong. Fig. 16.4-1 shows the stretch on a timeline.
Diagram
Fig. 16.4-1 — The Incubation Period On A Timeline A horizontal timeline arrow reading left to right, with three marked points: entry of germs into the body; germs multiplying inside (no symptoms felt); appearance of first symptoms. Bracket the stretch between the first and third points and label it incubation period. Label the stretch after the third point as period of illness.
Incubation periods differ enormously from one disease to another. Table 16.4 arranges some of them from the shortest to the longest.
16.5 Diseases Caused By Bacteria
Bacteria are single-celled organisms, far too small to see without a microscope. Most kinds are harmless and many are useful, but a few cause serious disease in humans.
You have already met the shapes bacteria take — rounded, rod-shaped, comma-shaped and spiral. The shape is often the first clue a laboratory uses in naming the germ found in a patient’s sample, as Fig. 16.5-1 shows.
Diagram
Fig. 16.5-1 — Shapes Of The Bacteria Of Some Common Diseases Line drawings of eight bacteria drawn to a comparable scale, each labelled with the disease it causes: chains of rounded cells (sore throat), paired oval cells (pneumonia), slender rods (tuberculosis), rods with flagella (typhoid), comma-shaped cells with a single flagellum (cholera), clustered paired cells (gonorrhoea), rods with a terminal spore (tetanus), coiled spiral cells (syphilis).
Three bacterial diseases are studied here in detail — cholera, typhoid and tuberculosis. Their main features are gathered together in Table 16.5 at the end of this discussion.
16.5.1 Cholera
Cholera attacks the intestinal tract. It is caused by a comma-shaped bacterium, Vibrio cholerae, and the incubation period is short — a few hours to about six days.
The patient suffers from vomiting and repeated loose motions. So much water is lost through them that there is very little urine, or none at all, and urea, a waste substance that the kidneys normally remove in urine, begins to accumulate in the body, where it acts as a poison.
This is why cholera can kill quickly if it is not treated promptly. The patient is not starved of food but of water, so treatment begins by putting water and salts back — in severe cases directly into the blood, as an injection of saline.
The cholera germ travels by what is called the faeco-oral route: it leaves the body in the faeces of a patient and enters another person through the mouth. Contaminated water and food carry it, and flies help by sitting on exposed excreta and then on uncovered food, the germs sticking to their spiny legs and body. FC 16.3 traces the journey.
Flowchart
FC 16.3 — The Faeco-Oral Route Of Cholera Germs in the faeces of a patient → carried by flies, dust or contaminated water → reach exposed food or drinking water → swallowed by a healthy person → germs multiply in the intestine → vomiting and loose motions → germs leave in the faeces again
Every step in that chain is also a place to break it. Good sanitation stops the germs at the start; killing flies removes a carrier; keeping food and cut fruit covered denies the flies a landing place; boiling drinking water and eating well-cooked food destroy any germs that get through. Immunization with an anti-cholera injection adds protection.
16.5.2 Typhoid Fever
Typhoid fever is caused by Salmonella typhi, a bacterium that carries flagella — fine whip-like threads that let it move — and attacks the intestines.
Its major symptom is a continuous fever which usually rises in the afternoon. Reddish eruptions appear on the chest and abdomen, and the patient has a severe headache. The incubation period is about 7 to 14 days, although it may be as long as 30 days.
The patient passes out the germs in the excreta, and from there flies and the direct contamination of food — particularly of milk — spread the disease to others.
Recovery takes about two to three weeks and leaves the patient very weak, and a relapse is not uncommon. Some people continue to carry the germs after they have recovered and feel perfectly well. Such a person is called a carrier, and can pass the disease to others without ever appearing ill.
Chloromycetin is a very effective antibiotic — a medicine that kills bacteria or stops them multiplying — in treating typhoid. Prevention rests on anti-typhoid inoculation, which should be repeated every year, together with proper sanitation and the control of flies.
16.5.3 Tuberculosis
Tuberculosis, or TB, is usually a disease of the lungs, though other parts including the brain, the kidney and the bones may also be affected. It is caused by a small rod-shaped bacterium, Mycobacterium tuberculosis.
The germ is spread by the sputum of infected persons, and may then be carried on air or dust to be breathed in by someone else. The incubation period is 2 to 10 weeks.
Once inside, the germs do not have matters their own way. The surrounding tissues build a kind of wall around them, called a tubercle, and hold them in. The germs are widespread and almost everyone receives them at some time, but most people overcome these minor infections without ever knowing it.
The disease becomes severe when the body’s resistance is low, which is why undernourishment and crowded living conditions matter so much in it. Treatment in the early stages is important. Streptomycin is a very effective antibiotic against tuberculosis; it works by inhibiting the growth of the bacteria.
Prevention is by BCG vaccination, which has been found useful in developing immunity to tuberculosis, and by isolating patients until they are no longer infectious.
Table 16.5 brings the three diseases together.
16.5.4 Other Bacterial Diseases
Several other diseases are caused by bacteria, and a few are worth knowing about because they are common, because they are preventable, or both.
Tetanus, also called lockjaw, is caused by Clostridium tetani. The germ is frequently present in soil and in horse and cow dung, and enters the body through deep wounds — even an ordinary cut from a fall on the ground can be a source. Its incubation period is 4 to 20 days, and in its last phase the muscles of the jaw go into spasm and hold the jaws tightly closed. It is prevented by previous vaccination, and an anti-toxin is given after a serious wound.
Diphtheria strikes young children. The germ, Corynebacterium diphtheriae, is spread by discharges from the throat of infected persons when they sneeze or cough. The incubation period is 2 to 5 days. It affects the throat and grows very rapidly, and it is prevented by inoculating a vaccine.
Whooping cough, or pertussis, also occurs mainly in children and is caused by the bacterium Bordetella pertussis. It spreads by discharges from the throat of an infected person, and its incubation period is 10 to 15 days. A bout of coughing ends in a sudden noisy intake of breath — the whoop that gives the disease its name. Vaccination prevents it.
These three are prevented together. The triple vaccine, also called DPT, is a combined vaccine given by injection to young babies to protect them from diphtheria, whooping cough (pertussis) and tetanus.
Pneumonia is a serious disease of the lungs caused by Streptococcus pneumoniae, spread by contact or by air, with an incubation period of 1 to 3 days. Antibiotics such as penicillin are very effective in curing it.
Leprosy is caused by Mycobacterium leprae. It spreads only after close and prolonged contact, its incubation period runs to several years, and it damages nerves, so that the affected part loses sensation; the injuries that follow a loss of feeling cause the deformity associated with the disease. It is completely curable with a combination of antibiotics, and a person under treatment is not a source of infection to others.
Syphilis and gonorrhoea are sexually transmitted diseases — diseases passed on through sexual contact with an infected person. Syphilis is caused by Treponema pallidum and gonorrhoea by Neisseria gonorrhoeae. Both begin with sores or discharge in the reproductive tract; untreated syphilis may later spread to other systems of the body and become dangerous. Both respond to antibiotics, and penicillin has long been used against syphilis. Table 16.6 summarises these diseases.
16.6 Diseases Caused By Protozoa
Protozoa are single-celled organisms, larger and far more complex than bacteria. A few kinds live as parasites in the human body, and the diseases they cause are among the most widespread in the warmer parts of the world.
16.6.1 Malaria
Malaria is caused by a protozoan called Plasmodium. It does not travel from person to person directly. It is carried by the bite of an infected female Anopheles mosquito, shown in Fig. 16.6-1.
Diagram
Fig. 16.6-1 — Female Anopheles Mosquito Side view of an adult female Anopheles mosquito resting on a skin surface, drawn with the body held at a slant to the surface; label the proboscis, antenna, palps (as long as the proboscis), compound eye, thorax, spotted wing, legs and abdomen. Add a note line: body held at an angle to the surface while resting.
The story begins with a mosquito that is not yet infected. When such a female mosquito bites a person who has malaria, the parasite is sucked in along with the person’s blood. Inside the mosquito’s body the parasite grows and multiplies within its gut.
That mosquito is now infected, and is known as a vector — an animal that carries a pathogen from one host to another without suffering the disease itself. When this vector bites a healthy person, it passes the infective stage of the parasite, along with its saliva, into that person’s bloodstream. FC 16.4 shows the full circle.
Flowchart
FC 16.4 — Transmission Cycle Of The Malarial Parasite Uninfected female Anopheles bites a person with malaria → parasites taken up with the blood → parasites grow and multiply in the mosquito’s gut → mosquito becomes an infected vector → bites a healthy person → infective stage passed in with the saliva → parasites multiply in the liver cells → parasites enter and destroy the red blood cells → chill and fever appear → another mosquito bites this person, and the cycle returns to the start
Inside the human body the parasite first grows and multiplies in the liver cells. It then goes on to infect and destroy the red blood cells. The incubation period is about three weeks.
The destruction of red blood cells happens in waves, and the symptoms follow those waves. A chill followed by high fever is repeated on the third or fourth day, and between the attacks the patient may feel almost well. Various drugs such as quinine, paludrin and camoquin are useful in treatment.
Malaria is common in hot and humid countries, because the mosquito breeds in standing water and needs warmth. Control therefore has two arms. The first is destruction of the mosquito at all its stages, including the eggs and larvae in stagnant water. The second is avoiding the bite itself, by using mosquito nets and repellents.
16.6.2 Amoebic Dysentery
Amoebic dysentery, also called amoebiasis, is caused by a kind of amoeba named Entamoeba histolytica. It lives in the large intestine and destroys its lining.
The damage produces diarrhoea with griping pain, and a discharge of mucus and sometimes blood in the stools. The incubation period is about one week.
Infection is by contamination of food, especially by flies that have settled on infected material and then on food. Proper sanitation, and protecting food from dust and flies, prevents the disease from spreading.
16.6.3 Sleeping Sickness
Sleeping sickness is caused by the flagellated protozoans Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense. It is transmitted through the bite of the tsetse fly, which is therefore its vector, in the way the Anopheles mosquito is the vector of malaria.
The disease runs in two phases. In the first, the parasites are in the blood and lymph, and the symptoms are fever, headache, itchiness and joint pains.
In the second phase the parasites reach the brain and the spinal cord, and this is called the neurological phase. The patient suffers from confusion, poor coordination, numbness and disturbed sleep — the sign that gives the disease its name. The disease can be fatal at this stage, so treatment matters early; drugs such as melarsoprol are administered.
16.7 Diseases Caused By Parasitic Worms
Bacteria and protozoa are single cells. The pathogens in this group are whole animals — worms, sometimes several centimetres long, living inside the human body and feeding at its expense. Worms of this kind are called helminths.
A parasite that lives inside the body of its host and takes its food from it is not trying to kill the host; a dead host is a dead home. That is why these diseases often develop slowly, weakening a person over months rather than striking in days.
16.7.1 Ascariasis
Ascariasis is caused by the common roundworm, Ascaris lumbricoides. It is an elongated cylindrical worm about the size of an earthworm, with the female slightly longer than the male, and the male recognisable by its slightly curved hind end. Fig. 16.7-1 shows both.
Diagram
Fig. 16.7-1 — Male And Female Roundworm, Ascaris lumbricoides External view of two roundworms lying side by side on a plain background: the female, about 18 cm long, with a straight tapering hind end; the male, about 12 cm long, with a curved hind end. Label female (external view, length 18 cm), male (external view, length 12 cm), mouth end and tail end of each.
The worms live in the intestines, where they absorb the food the host has already digested. The host does the work of digestion and the worm takes the meal.
The female lays several thousand eggs a day, and these pass out with the patient’s faeces. The eggs are scattered in the soil, and from the soil they reach the vegetables growing in the fields.
The rest follows from where the eggs are. Children playing on the ground pick up the microscopic eggs on their hands, and the infection occurs when they eat with unwashed hands, or when anyone eats unwashed raw vegetables.
16.7.2 Taeniasis
Taeniasis is caused by tapeworms. Two are common in humans: one, Taenia solium, is spread by eating infected pork, and a closely related species, Taenia saginata, by infected beef.
The tapeworm remains in the intestine, where it may reach a length of a metre or so. It absorbs most of the host’s digested food, and the patient becomes very weak.
The mature worm breaks off a small segment from its tail end, and this passes out with the faeces. Each such segment is packed with eggs. Fig. 16.7-2 shows the worm and the stages of its life history.
Diagram
Fig. 16.7-2 — Tapeworm And Its Life-History Stages Six labelled parts on one plate: A — adult tapeworm lodged in the intestine, shown as a long ribbon of segments within a cut-open length of intestine; B — the anchoring head or scolex, magnified, showing hooks and suckers; C — one mature segment containing eggs; D — a single egg; E — the six-hooked embryo within the egg; F — the bladderworm larval stage embedded in the flesh of a pig or cow. Label scolex, segments and each stage A to F.
If the eggs are accidentally swallowed by a pig or a cow, they hatch and form larval stages which settle in the muscles of that new host. When a person eats raw or imperfectly cooked pork or beef from such an animal, the worm matures in the human intestine. The human is the final host, and FC 16.5 closes the circle.
Flowchart
FC 16.5 — Life Cycle Of The Tapeworm Adult tapeworm in the human intestine → egg-filled segments break off and pass out in the faeces → eggs reach soil, water or fodder → swallowed by a pig or a cow → eggs hatch and larvae settle in the animal’s muscles as bladderworms → raw or undercooked pork or beef eaten by a person → larva develops into an adult worm in the intestine, and the cycle returns to the start
Notice where the chain can be broken most easily. Cooking meat thoroughly kills the larva in the muscle, and proper sanitation keeps human faeces away from the animals’ food and water.
16.7.3 Filariasis
Filariasis is a tropical disease also known as elephantiasis. It is caused by the filarial worm Wuchereria bancrofti, and is transmitted through the bite of the Culex mosquito.
These tiny worms lie in the lymphatic system and connective tissues of the body, mainly in the limbs. The lymphatic system is a network of fine vessels that drains the fluid which collects between the tissues and returns it to the blood.
The worms block that drainage. Fluid that should be carried away collects instead, the lymph glands and lymph vessels become inflamed, and the affected limb swells enormously. The legs come to resemble those of an elephant, and this is where the name elephantiasis comes from.
The symptoms are the enlargement of the limbs and ankles, with fever and chills in acute cases. Treatment involves eradication of the vector, together with antibiotics and anti-inflammatory analgesics to control the infection and the swelling.
16.9 Summary
A disease is a departure from normal health, caused by a structural or functional disorder of the body. Some diseases are mild and clear up on their own; others are severe and may be fatal.
Disease was once believed to be the work of gods, spirits or sorcery. Science treats every disease as having a definite cause, which can be found — and this is what makes prevention and cure possible.
World Health Day is observed on 7 April every year.
Diseases can be classified on four bases: the extent of their occurrence, their communicability, the kind of pathogen causing them, and the kind of agent that transmits them.
An endemic disease is confined to one particular area and attacks comparatively few people there, for example yellow fever in certain African countries and goitre in the sub-Himalayan regions.
An epidemic disease breaks out suddenly, spreads from place to place, and affects large numbers of people at the same time, as plague did in India in 1994.
A pandemic disease is distributed widely over the world, across many countries; AIDS and COVID-19 are examples.
A sporadic disease occurs as scattered individual cases, with no regular pattern linking them.
Non-communicable or non-infectious diseases involve no germ and cannot pass from one person to another. Diabetes, colour blindness, heart attack and beri-beri are examples.
Non-infectious diseases include nutritional deficiency diseases, metabolic diseases, genetic diseases, allergies, degenerative or ageing diseases, mental illnesses and cancers.
Cancer is an abnormal, uncontrolled multiplication of cells; the mass formed is a tumour, and any agent that causes cancer is a carcinogen.
Communicable or infectious diseases are caused by germs, which reach a healthy person from an infected one. A disease-causing germ is a pathogen, and its transmission from person to person is infection.
Pathogens belong to four groups — bacteria, viruses, protozoa and helminths (worms) — and the group decides the treatment.
The incubation period is the period between the entry of germs and the appearance of the first symptoms. It is a few hours to six days in cholera, and may run to twelve years or more in HIV infection.
Cholera is caused by Vibrio cholerae, attacks the intestinal tract, and spreads by the faeco-oral route through contaminated food and water, with flies as carriers. Loss of water is the danger, and saline replaces it.
Typhoid fever is caused by Salmonella typhi, spreads through contaminated food and milk, and shows a continuous fever rising in the afternoon with reddish eruptions on the chest. Some people remain carriers after recovery.
Tuberculosis is caused by Mycobacterium tuberculosis, spreads by the sputum of infected persons through air and dust, and mainly attacks the lungs. Streptomycin treats it and BCG vaccination prevents it.
Other bacterial diseases include tetanus, diphtheria, whooping cough, pneumonia, leprosy, syphilis and gonorrhoea. The triple vaccine, DPT, protects young children against diphtheria, whooping cough and tetanus together.
Malaria is caused by the protozoan Plasmodium and is carried by the female Anopheles mosquito, which acts as a vector. The parasite multiplies in the liver cells and then destroys the red blood cells, producing chill and fever every third or fourth day.
Amoebic dysentery is caused by Entamoeba histolytica, which destroys the lining of the large intestine, producing diarrhoea with mucus and sometimes blood.
Sleeping sickness is caused by Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense, transmitted by the tsetse fly, and reaches a neurological phase that can be fatal.
Ascariasis is caused by the round worm Ascaris lumbricoides, which absorbs digested food in the intestine. Its eggs pass out in the faeces, scatter in the soil, and return through unwashed hands and unwashed raw vegetables.
Taeniasis is caused by tapeworms, which anchor in the intestine and absorb the host’s digested food. Egg-filled segments pass out in the faeces, are taken up by a pig or cow, and return to humans in raw or undercooked meat.
Filariasis, or elephantiasis, is caused by the filarial worm Wuchereria bancrofti and transmitted by the Culex mosquito. The worms block lymph circulation, so fluid collects and the limb swells.
Viruses consist of a nucleic acid and a protein coat, are visible only under the electron microscope, and can multiply only inside a host cell, which they command and usually kill. They can be crystallized and are regarded as a connecting link between living and non-living things.
HIV attacks the immune system, so that ordinary infections become serious; AIDS is the last stage of HIV infection. The virus spreads by sexual contact, contaminated blood, mother to child, and shared needles — not by shaking hands, eating together or sharing toilets.
Chicken pox is caused by Varicella zoster and spreads by close contact; hepatitis, caused by five strains of virus, inflames the liver; COVID-19 is caused by SARS-CoV-2 and spreads through the air by the respiratory route.
Viruses differ from bacteria in size, in having no cell and no metabolism, in being unable to grow or divide by themselves, and in being able to be crystallized.
Flowchart
FC 16.7 — The Chapter In One Chain Disease → non-communicable (no germ; nutritional, metabolic, genetic, allergic, degenerative, mental illness, cancer) or communicable (caused by a pathogen) → pathogen is a bacterium (cholera, typhoid, tuberculosis), a virus (AIDS, chicken pox, hepatitis, COVID-19), a protozoan (malaria, amoebic dysentery, sleeping sickness) or a helminth (ascariasis, taeniasis, filariasis) → germ enters the body → incubation period → symptoms appear → spread checked by sanitation, vector control, vaccination and treatment
16.10 Key Terms
Antibiotic — a medicine that kills bacteria or stops them from multiplying inside the body.
Carcinogen — any agent, such as tobacco tar or certain radiations, that causes cancer.
Carrier — a person who continues to harbour and pass on the germs of a disease after recovering, without appearing ill.
Communicable (infectious) disease — a disease caused by a germ, which can pass from an infected person to a healthy person.
Disease — a departure from normal health, caused by a structural or functional disorder of the body.
Endemic disease — a disease confined to one particular area, where it attacks comparatively few people.
Epidemic disease — a disease that breaks out suddenly, spreads from place to place, and affects large numbers of people at the same time.
HIV-positive — describing a person in whose body HIV has been detected, whether or not any symptoms have appeared.
Host — the organism in or on which a parasite lives and from which it takes its food.
Incubation period — the period between the entry of germs into the body and the appearance of the first symptoms.
Infection — the entry of pathogens into the body and their transmission from one person to another.
Non-communicable (non-infectious) disease — a disease that involves no germ and cannot spread from one person to another.
Pandemic disease — a disease distributed widely over the world, across many countries and continents.
Pathogen — a disease-producing organism, such as a bacterium, virus, protozoan or parasitic worm.
Quarantine — keeping a person who may have been exposed to an infection away from others during the incubation period, to prevent spread.
Sexually transmitted diseases — diseases passed from one person to another through sexual contact, such as syphilis and gonorrhoea.
Sporadic disease — a disease that occurs as scattered individual cases, with no regular pattern linking them.
Triple vaccine (DPT) — a combined vaccine given to young babies to protect them against diphtheria, whooping cough (pertussis) and tetanus.
Tumour — a mass of cells formed by uncontrolled cell division.
Vector — an animal that carries a pathogen from one host to another without suffering the disease itself.
Virus — an extremely small particle of nucleic acid and protein that can multiply only inside a living host cell.
World Health Day — 7 April, observed each year to make people conscious of staying healthy and disease-free.
16.11 Points To Ponder
The tapeworm and the malarial parasite both live at the expense of a human host, yet the tapeworm rarely kills while the malarial parasite often does. What might make a parasite that harms its host less severely the more successful one in the long run?
Smallpox has been wiped off the Earth, but malaria and cholera have not, although all three have been known for centuries. What features of a disease decide whether it can be eradicated completely?
A very long incubation period, as in HIV infection, gives a person years of ordinary health before any illness appears. Why does this make the disease harder to control than one whose symptoms appear within days?
Diseases such as goitre, diabetes and colour blindness cannot be caught from another person, yet the number of people living with some of them keeps rising. If no germ is spreading, what else could be spreading?
Antibiotics act on bacteria but not on viruses. Working from what a virus is and what an antibiotic does, suggest why one kind of germ can be attacked with medicine far more easily than the other.
Every disease in this chapter travels by a definite route — air, water, food, an insect, or direct contact. If you could close only one of these routes completely for the whole country, which would save the most lives, and what would you need to know before deciding?
16.12 Objective Questions
- The period between the entry of germs and the appearance of the first symptoms is called the: (1 mark)
- infective period
- incubation period
- recovery period
- latent stage
- A disease found only in one particular region, affecting a small number of people, is said to be: (1 mark)
- epidemic
- pandemic
- endemic
- sporadic
- The pathogen of sleeping sickness is carried by: (1 mark)
- the Culex mosquito
- the housefly
- the tsetse fly
- the female Anopheles mosquito
- Which of the following is a helminthic disease? (1 mark)
- Amoebic dysentery
- Filariasis
- Hepatitis
- Diphtheria
- A virus can be crystallized and stored for an indefinite period. This supports the view that a virus is: (1 mark)
- a living organism
- a non-living thing
- a single-celled organism
- a bacterium
- Choose the correct option. (1 mark) Assertion (A): A person may pass on an infectious disease while feeling perfectly well. Reason (R): During the incubation period the germs are multiplying inside the body without producing symptoms.
- 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.
- Choose the correct option. (1 mark) Assertion (A): Antibiotics are of no use against COVID-19. Reason (R): Antibiotics act on the cell processes of bacteria, and a virus has no cell of its own.
- 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.
- Match the disease in Column I with its causative group in Column II. (4 marks)
| Column I | Column II |
|---|---|
| a. Taeniasis | i. Bacterium |
| b. Chicken pox | ii. Protozoan |
| c. Amoebic dysentery | iii. Virus |
| d. Tuberculosis | iv. Helminth |
- Complete each sentence with one suitable word. (5 marks)
- An animal that carries a pathogen without suffering the disease is called a ________.
- A mass of cells produced by uncontrolled cell division is called a ________.
- The germ of cholera travels by the ________ route.
- The combined vaccine given to babies against diphtheria, whooping cough and tetanus is the ________ vaccine.
- Filariasis is caused by the worm ________.
- State whether each statement is true or false. (4 marks)
- AIDS can spread by eating with a person who is HIV-positive.
- Beri-beri is a communicable disease.
- A carrier of typhoid may feel completely healthy.
- Viruses can multiply outside a living cell.
- Name the following. (5 marks)
- The bacterium that causes typhoid fever.
- The mosquito that transmits filariasis.
- The stage of HIV infection at which the body’s defences have failed.
- The vaccination used to develop immunity against tuberculosis.
- The protozoan that damages the lining of the large intestine.
- In each set, pick the odd one out and give a reason for your choice. (4 marks)
- Cholera, typhoid, tuberculosis, malaria
- Scurvy, kwashiorkor, night-blindness, chicken pox
- Ascariasis, taeniasis, filariasis, hepatitis
- Endemic, epidemic, pandemic, bacterial
16.13 Check Your Concepts
Define a disease, and state the two kinds of disorder that can produce one. (2 marks)
Give a reason: a person recovering from typhoid may still be a danger to others. (2 marks)
Name the four groups of pathogens, giving one disease caused by each. (2 marks)
Differentiate between an endemic and an epidemic disease, stating the basis of difference. (3 marks)
Give reasons for each of the following: (3 marks)
- Drinking water is boiled during a cholera outbreak.
- Meat must be cooked thoroughly.
- Salt sold in India is fortified with iodine.
Explain, with one example, why a disease carried by a vector cannot spread beyond the region where the vector lives. (3 marks)
Why is a virus described as a connecting link between living and non-living things? Give one feature on each side. (3 marks)
Differentiate between a virus and a bacterium, stating any four bases of difference. (4 marks)
Describe how the malarial parasite passes from an infected person to a healthy person, naming the vector and the two organs affected inside the human body. (4 marks)
What would happen if a patient with tuberculosis stopped taking the medicines as soon as the cough improved? Explain your answer. (4 marks)
Trace the journey of the cholera germ from the faeces of a patient to the intestine of a healthy person, and state one preventive measure that could break each step of that journey. (5 marks)
A student says that AIDS can be caught by sharing a desk with an HIV-positive classmate. State whether this is correct, list the ways in which HIV actually spreads, and explain why such beliefs are harmful. (5 marks)
16.14 Figure-Based Questions
- Study Fig. 16.4-1. (3 marks)
- Name the stretch of time marked on the timeline between the entry of germs and the first symptoms.
- Why does the person feel healthy during this stretch?
- Name one disease in which this stretch is only a few hours, and one in which it runs into years.
- Fig. 16.6-1 shows an insect that transmits a protozoan disease. (3 marks)
- Identify the insect and state its sex.
- Name the disease it transmits and the pathogen involved.
- State one visible feature by which it is drawn here.
- Fig. 16.7-1 shows two roundworms. (4 marks)
- Identify the organism and name the disease it causes.
- State which of the two is the female and give the reason.
- How do the eggs of this worm reach the human mouth?
- State one way of preventing this infection.
- Fig. 16.7-2 shows a parasite and the stages of its life history. (5 marks)
- Name the parasite and the part of the human body in which the adult lives.
- Name the part labelled B and state its function.
- Name the animals in whose muscles the larval stage settles.
- How does the parasite enter the human body?
- Redraw the anchoring head and label two structures on it.
- Fig. 16.8-1 shows a virus particle. (4 marks)
- Identify the virus and name the disease it finally produces.
- Name the two labelled parts other than the surface projections.
- Which system of the body does this virus attack?
- Redraw the particle and label the genetic material.