NCERT-Books-for-class-9-Science-Chapter-13

C 13hapter WHY DO WE FALL ILL Activity _____________ 13.1 the kidney is filtering urine, the brain is thinking. • We have all heard of the earthquakes in Latur, Bhuj, Kashmir etc. or the All these activities are interconnected. For cyclones that attack the coastal example, if the kidneys are not filtering urine, regions. Think of as many different poisonous substances will accumulate. Under ways as possible in which people’s such conditions, the brain will not be able to health would be affected by such a think properly. For all these interconnected disaster if it took place in our activities, energy and raw material are needed neighbourhood. from outside the body. In other words, food is a necessity for cell and tissue functions. • How many of these ways we can think of Anything that prevents proper functioning of are events that would occur when the cells and tissues will lead to a lack of proper disaster is actually happening? activity of the body. • How many of these health-related events It is in this context that we will now look would happen long after the actual at the notions of health and disease. disaster, but would still be because of the disaster? 13.1 Health and its Failure • Why would one effect on health fall into 13.1.1 THE SIGNIFICANCE OF ‘HEALTH’ the first group, and why would another fall into the second group? We have heard the word ‘health’ used quite frequently all around us. We use it ourselves When we do this exercise, we realise that as well, when we say things like ‘my health and disease in human communities grandmother’s health is not good’. Our are very complex issues, with many teachers use it when they scold us saying ‘this interconnected causes. We also realise that is not a healthy attitude’. What does the word the ideas of what ‘health’ and ‘disease’ mean ‘health’ mean? are themselves very complicated. When we ask what causes diseases and how we prevent If we think about it, we realise that it them, we have to begin by asking what these always implies the idea of ‘being well’. We can notions mean. think of this well-being as effective functioning. For our grandmothers, being able We have seen that cells are the basic units to go out to the market or to visit neighbours of living beings. Cells are made of a variety of is ‘being well’, and not being able to do such chemical substances – proteins, carbo- things is ‘poor health’. Being interested in hydrates, fats or lipids, and so on. Although following the teaching in the classroom so that the pictures look quite static, in reality the we can understand the world is called a living cell is a dynamic place. Something or ‘healthy attitude’; while not being interested the other is always happening. Cells move is called the opposite. ‘Health’ is therefore a from place to place. Even in cells that do not state of being well enough to function well move, there is repair going on. New cells are physically, mentally and socially. being made. In our organs or tissues, there are various specialised activities going on – the heart is beating, the lungs are breathing,

13.1.2 PERSONAL AND COMMUNITY ISSUES We need food for health, and this food will have to be earned by doing work. For this, BOTH MATTER FOR HEALTH the opportunity to do work has to be available. Good economic conditions and jobs are If health means a state of physical, mental therefore needed for individual health. and social well-being, it cannot be something that each one of us can achieve entirely on We need to be happy in order to be truly our own. The health of all organisms will healthy, and if we mistreat each other and depend on their surroundings or their are afraid of each other, we cannot be happy environment. The environment includes the or healthy. Social equality and harmony are physical environment. So, for example, health therefore necessary for individual health. We is at risk in a cyclone in many ways. can think of many other such examples of connections between community issues and But even more importantly, human beings individual health. live in societies. Our social environment, therefore, is an important factor in our 13.1.3 DISTINCTIONS BETWEEN ‘HEALTHY’ individual health. We live in villages, towns AND ‘DISEASE-FREE’ or cities. In such places, even our physical environment is decided by our social If this is what we mean by ‘health’, what do environment. we mean by ‘disease’? The word is actually self-explanatory – we can think of it as Consider what would happen if no agency ‘disease’ – disturbed ease. Disease, in other is ensuring that garbage is collected and words, literally means being uncomfortable. disposed. What would happen if no one takes However, the word is used in a more limited responsibility for clearing the drains and meaning. We talk of disease when we can find ensuring that water does not collect in the a specific and particular cause for discomfort. streets or open spaces? This does not mean that we have to know the absolute final cause; we can say that So, if there is a great deal of garbage someone is suffering from diarrhoea without thrown in our streets, or if there is open drain- knowing exactly what has caused the loose water lying stagnant around where we live, motions. the possibility of poor health increases. Therefore, public cleanliness is important for We can now easily see that it is possible individual health. to be in poor health without actually suffering from a particular disease. Simply not being Activity _____________ 13.2 diseased is not the same as being healthy. ‘Good health’ for a dancer may mean being • Find out what provisions are made by able to stretch his body into difficult but your local authority (panchayat/ graceful positions. On the other hand, good municipal corporation) for the supply health for a musician may mean having enough of clean drinking water. breathing capacity in his/her lungs to control the notes from his/her flute. To have the • Are all the people in your locality able opportunity to realise the unique potential to access this? in all of us is also necessary for real health. Activity _____________ 13.3 So, we can be in poor health without there being a simple cause in the form of an • Find out how your local authority identifiable disease. This is the reason why, manages the solid waste generated in when we think about health, we think about your neighbourhood. societies and communities. On the other hand, when we think about disease, we think • Are these measures adequate? about individual sufferers. • If not, what improvements would you suggest? • What could your family do to reduce the amount of solid waste generated during a day/week? WHY DO WE FALL ILL 177

Q uestions 13.2.2 ACUTE AND CHRONIC DISEASES 1. State any two conditions essential for good health. The manifestations of disease will be different 2. State any two conditions depending on a number of factors. One of the essential for being free of disease. most obvious factors that determine how we 3. Are the answers to the above perceive the disease is its duration. Some questions necessarily the same diseases last for only very short periods of or different? Why? time, and these are called acute diseases. We all know from experience that the common 13.2 Disease and Its Causes cold lasts only a few days. Other ailments can last for a long time, even as much as a lifetime, 13.2.1 WHAT DOES DISEASE LOOK LIKE? and are called chronic diseases. An example is the infection causing elephantiasis, which Let us now think a little more about diseases. is very common in some parts of India. In the first place, how do we know that there is a disease? In other words, how do we know Activity _____________ 13.4 that there is something wrong with the body? There are many tissues in the body, as we • Survey your neighbourhood to find out: have seen in Chapter 6. These tissues make (1) how many people suffered from up physiological systems or organ systems acute diseases during the last three that carry out body functions. Each of the months, organ systems has specific organs as its parts, (2) how many people developed chronic and it has particular functions. So, the diseases during this same period, digestive system has the stomach and (3) and finally, the total number of intestines, and it helps to digest food taken people suffering from chronic in from outside the body. The musculoskeletal diseases in your neighbourhood. system, which is made up of bones and muscles, holds the body parts together and • Are the answers to questions (1) and helps the body move. (2) different? When there is a disease, either the • Are the answers to questions (2) and functioning or the appearance of one or more (3) different? systems of the body will change for the worse. These changes give rise to symptoms and • What do you think could be the reason signs of disease. Symptoms of disease are the for these differences? What do you think things we feel as being ‘wrong’. So we have a would be the effect of these differences headache, we have cough, we have loose on the general health of the population? motions, we have a wound with pus; these are all symptoms. These indicate that there 13.2.3 CHRONIC DISEASES AND POOR may be a disease, but they don’t indicate what the disease is. For example, a headache may HEALTH mean just examination stress or, very rarely, it may mean meningitis, or any one of a dozen As we can imagine, acute and chronic different diseases. diseases have different effects on our health. Any disease that causes poor functioning of Signs of disease are what physicians will some part of the body will affect our general look for on the basis of the symptoms. Signs health as well. This is because all functions will give a little more definite indication of of the body are necessary for general health. the presence of a particular disease. But an acute disease, which is over very soon, Physicians will also get laboratory tests done will not have time to cause major effects on to pinpoint the disease further. general health, while a chronic disease will do so. As an example, think about a cough and cold, which all of us have from time to time. Most of us get better and become well within a week or so. And there are no bad effects on 178 SCIENCE

our health. We do not lose weight, we do not would not lead to loose motions. But they do become short of breath, we do not feel tired become contributory causes of the disease. all the time because of a few days of cough and cold. But if we get infected with a chronic Why was there no clean drinking water disease such as tuberculosis of the lungs, for the baby? Perhaps because the public then being ill over the years does make us services are poor where the baby’s family lose weight and feel tired all the time. lives. So, poverty or lack of public services become third-level causes of the baby’s We may not go to school for a few days if disease. we have an acute disease. But a chronic disease will make it difficult for us to follow It will now be obvious that all diseases what is being taught in school and reduce will have immediate causes and contributory our ability to learn. In other words, we are causes. Also, most diseases will have many likely to have prolonged general poor health causes, rather than one single cause. if we have a chronic disease. Chronic diseases therefore, have very drastic long-term effects 13.2.5 INFECTIOUS AND NON-INFECTIOUS on people’s health as compared to acute diseases. CAUSES 13.2.4 CAUSES OF DISEASES As we have seen, it is important to keep public health and community health factors in mind What causes disease? When we think about when we think about causes of diseases. We causes of diseases, we must remember that can take that approach a little further. It is there are many levels of such causes. Let us useful to think of the immediate causes of look at an example. If there is a baby suffering disease as belonging to two distinct types. One from loose motions, we can say that the cause group of causes is the infectious agents, of the loose motions is an infection with a mostly microbes or micro-organisms. virus. So the immediate cause of the disease Diseases where microbes are the immediate is a virus. causes are called infectious diseases. This is because the microbes can spread in the But the next question is – where did the community, and the diseases they cause will virus come from? Suppose we find that the spread with them. virus came through unclean drinking water. But many babies must have had this unclean Things to ponder drinking water. So, why is it that one baby developed loose motions when the other 1. Do all diseases spread to people babies did not? coming in contact with a sick person? One reason might be that this baby is not 2. What are the diseases that are not healthy. As a result, it might be more likely spreading? to have disease when exposed to risk, whereas healthier babies would not. Why is the baby 3. How would a person develop those not healthy? Perhaps because it is not well diseases that don’t spread by contact nourished and does not get enough food. So, with a sick person? lack of good nourishment becomes a second- level cause of the disease the baby is suffering On the other hand, there are also diseases from. Further, why is the baby not well that are not caused by infectious agents. Their nourished? Perhaps because it is from a causes vary, but they are not external causes household which is poor. like microbes that can spread in the community. Instead, these are mostly It is also possible that the baby has some internal, non-infectious causes. genetic difference that makes it more likely to suffer from loose motions when exposed For example, some cancers are caused by to such a virus. Without the virus, the genetic genetic abnormalities. High blood pressure difference or the poor nourishment alone can be caused by excessive weight and lack of exercise. You can think of many other diseases where the immediate causes will not be infectious. WHY DO WE FALL ILL 179

Peptic ulcers and the Nobel prize The ways in which diseases spread, and the ways in which they can be treated and For many years, everybody used to think prevented at the community level would be that peptic ulcers, which cause acidity– different for different diseases. This would related pain and bleeding in the stomach depend a lot on whether the immediate and duodenum, were because of lifestyle causes are infectious or non-infectious. reasons. Everybody thought that a stressful life led to a lot of acid secretion Q uestions in the stomach, and eventually caused 1. List any three reasons why you peptic ulcers. would think that you are sick and ought to see a doctor. If only one Then two Australians made a discovery of these symptoms were present, that a bacterium, Helicobacter pylori, was would you still go to the doctor? responsible for peptic ulcers. Robin Warren Why or why not? (born 1937), a pathologist from Perth, 2. In which of the following case do Australia, saw these small curved bacteria you think the long-term effects on in the lower part of the stomach in many your health are likely to be most patients. He noticed that signs of unpleasant? inflammation were always present around • if you get jaundice, these bacteria. Barry Marshall (born 1951), • if you get lice, a young clinical fellow, became interested • if you get acne. in Warren’s findings and succeeded in Why? cultivating the bacteria from these sources. 13.3 Infectious Diseases In treatment studies, Marshall and Warren showed that patients could be 13.3.1 INFECTIOUS AGENTS cured of peptic ulcer only when the bacteria were killed off from the stomach. We have seen that the entire diversity seen in Thanks to this pioneering discovery by the living world can be classified into a few Marshall and Warren, peptic ulcer disease groups. This classification is based on is no longer a chronic, frequently disabling common characteristics between different condition, but a disease that can be cured organisms. Organisms that can cause disease by a short period of treatment with are found in a wide range of such categories antibiotics. of classification. Some of them are viruses, some are bacteria, some are fungi, some are For this achievement, Marshall and single-celled animals or protozoans. Some Warren (seen in the picture) received the diseases are also caused by multicellular Nobel prize for physiology and medicine organisms, such as worms of different kinds. in 2005. 180 SCIENCE

Fig. 13.1(a): Picture of SARS viruses coming out (see Fig. 13.1(d): Picture of Leishmania, the protozoan arrows for examples) of the surface of organism that causes kala-azar. The an infected cell. The white scale line organisms are oval-shaped, and each represents 500 nanometres, which is has one long whip-like structure. One half a micrometre, which is one- organism (arrow) is dividing, while a cell thousandth of a millimetre. The scale line of the immune system (lower right) has gives us an idea of how small the things gripped on the two whips of the dividing we are looking at are. organism and is sending cell processes Courtesy: Emerging Infectious up to eat up the organism. The immune Deseases, a journal of CDC, U.S. cell is about ten micrometres in diameter. Fig. 13.1(b): Picture of staphylococci, the bacteria which can cause acne. The scale of the image is indicated by the line at top left, which is 5 micrometres long. Fig. 13.1(c): Picture of Trypanosoma, the protozoan Fig. 13.1(e): Picture of an adult roundworm (Ascaris organism responsible for sleeping lumbricoides is the technical name) from sickness. The organism is lying next to the small intestine. The ruler next to it a saucer-shaped red blood cell to give shows four centimetres to give us an an idea of the scale. idea of the scale. Copyright: Oregon Health and Science University, U.S. WHY DO WE FALL ILL 181

Common examples of diseases caused by But viruses do not use these pathways at viruses are the common cold, influenza, all, and that is the reason why antibiotics do dengue fever and AIDS. Diseases like typhoid not work against viral infections. If we have a fever, cholera, tuberculosis and anthrax are common cold, taking antibiotics does not caused by bacteria. Many common skin reduce the severity or the duration of the infections are caused by different kinds of disease. However, if we also get a bacterial fungi. Protozoan microbes cause many infection along with the viral cold, taking familiar diseases, such as malaria and kala- antibiotics will help. Even then, the antibiotic azar. All of us have also come across intestinal will work only against the bacterial part of worm infections, as well as diseases like the infection, not the viral infection. elephantiasis caused by diffferent species of worms. Activity _____________ 13.5 Why is it important that we think of these • Find out how many of you in your class categories of infectious agents? The answer had cold/cough/fever recently. is that these categories are important factors in deciding what kind of treatment to use. • How long did the illness last? Members of each one of these groups – • How many of you took antibiotics (ask viruses, bacteria, and so on – have many biological characteristics in common. your parents if you had antibiotics)? • How long were those who took All viruses, for example, live inside host cells, whereas bacteria very rarely do. Viruses, antibiotics ill? bacteria and fungi multiply very quickly, while • How long were those who didn’t take worms multiply very slowly in comparison. Taxonomically, all bacteria are closely related antibiotics ill? to each other than to viruses and vice versa. • Is there a difference between these two This means that many important life processes are similar in the bacteria group groups? but are not shared with the virus group. As a • If yes, why? If not, why not? result, drugs that block one of these life processes in one member of the group is likely 13.3.2 MEANS OF SPREAD to be effective against many other members of the group. But the same drug will not work How do infectious diseases spread? Many against a microbe belonging to a different microbial agents can commonly move from group. an affected person to someone else in a variety of ways. In other words, they can be As an example, let us take antibiotics. ‘communicated’, and so are also called They commonly block biochemical pathways communicable diseases. important for bacteria. Many bacteria, for example, make a cell-wall to protect Such disease-causing microbes can themselves. The antibiotic penicillin blocks spread through the air. This occurs through the bacterial processes that build the cell- the little droplets thrown out by an infected wall. As a result, the growing bacteria become person who sneezes or coughs. Someone unable to make cell-walls, and die easily. standing close by can breathe in these Human cells don’t make a cell-wall anyway, droplets, and the microbes get a chance to so penicillin cannot have such an effect on start a new infection. Examples of such us. Penicillin will have this effect on any diseases spread through the air are the bacteria that use such processes for making common cold, pneumonia and tuberculosis. cell-walls. Similarly, many antibiotics work against many species of bacteria rather than We all have had the experience of sitting simply working against one. near someone suffering from a cold and catching it ourselves. Obviously, the more crowded our living conditions are, the more likely it is that such airborne diseases will spread. 182 SCIENCE

inevitable that many diseases will be transmitted by other animals. These animals carry the infecting agents from a sick person to another potential host. These animals are thus the intermediaries and are called vectors. The commonest vectors we all know are mosquitoes. In many species of mosquitoes, the females need highly nutritious food in the form of blood in order to be able to lay mature eggs. Mosquitoes feed on many warm-blooded animals, including us. In this way, they can transfer diseases from person to person. Fig. 13.2: Air-transmitted diseases are easier to catch the closer we are to the infected person. However, in closed areas, the droplet nuclei recirculate and pose a risk to everybody. Overcrowded and poorly ventilated housing is therefore a major factor in the spread of airborne diseases. Diseases can also be spread through Fig. 13.3: Common methods of transmission of water. This occurs if the excreta from someone suffering from an infectious gut disease, such diseases. as cholera, get mixed with the drinking water used by people living nearby. The cholera- 13.3.3 ORGAN-SPECIFIC AND TISSUE- causing microbes will enter new hosts through the water they drink and cause SPECIFIC MANIFESTATIONS disease in them. Such diseases are much more likely to spread in the absence of safe The disease-causing microbes enter the body supplies of drinking water. through these different means. Where do they go then? The body is very large when The sexual act is one of the closest compared to the microbes. So there are many physical contact two people can have with possible places, organs or tissues, where they each other. Not surprisingly, there are could go. Do all microbes go to the same tissue microbial diseases such as syphilis or AIDS or organ, or do they go to different ones? that are transmitted by sexual contact from one partner to the other. However, such Different species of microbes seem to have sexually transmitted diseases are not spread evolved to home in on different parts of the by casual physical contact. Casual physical body. In part, this selection is connected to contacts include handshakes or hugs or their point of entry. If they enter from the air sports, like wrestling, or by any of the other via the nose, they are likely to go to the lungs. ways in which we touch each other socially. Other than the sexual contact, the AIDS virus can also spread through blood-to-blood contact with infected people or from an infected mother to her baby during pregnancy or through breast feeding. We live in an environment that is full of many other creatures apart from us. It is WHY DO WE FALL ILL 183

This is seen in the bacteria causing minor gut infection can produce major tuberculosis. If they enter through the mouth, diarrhoea with blood loss. Ultimately, it is they can stay in the gut lining like typhoid- these other infections that kill people causing bacteria. Or they can go to the liver, suffering from HIV-AIDS. like the viruses that cause jaundice. It is also important to remember that the But this needn’t always be the case. An severity of disease manifestations depend on infection like HIV, that comes into the body the number of microbes in the body. If the via the sexual organs, will spread to lymph number of microbes is very small, the disease nodes all over the body. Malaria-causing manifestations may be minor or unnoticed. microbes, entering through a mosquito bite, But if the number is of the same microbe will go to the liver, and then to the red blood large, the disease can be severe enough to be cells. The virus causing Japanese life-threatening. The immune system is a encephalitis, or brain fever, will similarly enter major factor that determines the number of through a mosquito bite. But it goes on to microbes surviving in the body. We shall look infect the brain. into this aspect a little later in the chapter. The signs and symptoms of a disease will 13.3.4 PRINCIPLES OF TREATMENT thus depend on the tissue or organ which the microbe targets. If the lungs are the What are the steps taken by your family when targets, then symptoms will be cough and you fall sick? Have you ever thought why you breathlessness. If the liver is targeted, there sometimes feel better if you sleep for some will be jaundice. If the brain is the target, we time? When does the treatment involve will observe headaches, vomiting, fits or medicines? unconsciousness. We can imagine what the symptoms and signs of an infection will be if Based on what we have learnt so far, it we know what the target tissue or organ is, would appear that there are two ways to treat and the functions that are carried out by this an infectious disease. One would be to reduce tissue or organ. the effects of the disease and the other to kill the cause of the disease. For the first, we can In addition to these tissue-specific effects provide treatment that will reduce the of infectious disease, there will be other symptoms. The symptoms are usually common effects too. Most of these common because of inflammation. For example, we can effects depend on the fact that the body’s take medicines that bring down fever, reduce immune system is activated in response to pain or loose motions. We can take bed rest so infection. An active immune system recruits that we can conserve our energy. This will many cells to the affected tissue to kill off the enable us to have more of it available to focus disease-causing microbes. This recruitment on healing. process is called inflammation. As a part of this process, there are local effects such as But this kind of symptom-directed swelling and pain, and general effects such treatment by itself will not make the infecting as fever. microbe go away and the disease will not be cured. For that, we need to be able to kill off In some cases, the tissue-specificity of the the microbes. infection leads to very general-seeming effects. For example, in HIV infection, the How do we kill microbes? One way is to virus goes to the immune system and use medicines that kill microbes. We have seen damages its function. Thus, many of the earlier that microbes can be classified into effects of HIV-AIDS are because the body can different categories. They are viruses, bacteria, no longer fight off the many minor infections fungi or protozoa. Each of these groups of that we face everyday. Instead, every small organisms will have some essential cold can become pneumonia. Similarly, a biochemical life process which is peculiar to 184 SCIENCE

that group and not shared with the other we can get some easy answers. For airborne groups. These processes may be pathways for microbes, we can prevent exposure by the synthesis of new substances or respiration. providing living conditions that are not overcrowded. For water-borne microbes, we These pathways will not be used by us can prevent exposure by providing safe either. For example, our cells may make new drinking water. This can be done by treating substances by a mechanism different from the water to kill any microbial contamination. that used by bacteria. We have to find a drug For vector-borne infections, we can provide that blocks the bacterial synthesis pathway clean environments. This would not, for without affecting our own. This is what is example, allow mosquito breeding. In other achieved by the antibiotics that we are all words, public hygiene is one basic key to the familiar with. Similarly, there are drugs that prevention of infectious diseases. kill protozoa such as the malarial parasite. In addition to these issues that relate to One reason why making anti-viral the environment, there are some other general medicines is harder than making anti- principles to prevent infectious diseases. To bacterial medicines is that viruses have few appreciate those principles, let us ask a biochemical mechanisms of their own. They question we have not looked at so far. enter our cells and use our machinery for Normally, we are faced with infections their life processes. This means that there everyday. If someone is suffering from a cold are relatively few virus-specific targets to aim and cough in the class, it is likely that the at. Despite this limitation, there are now children sitting around will be exposed to the effective anti-viral drugs, for example, the infection. But all of them do not actually suffer drugs that keep HIV infection under control. from the disease. Why not? 13.3.5 PRINCIPLES OF PREVENTION This is because the immune system of our body is normally fighting off microbes. We All of what we have talked about so far deals have cells that specialise in killing infecting with how to get rid of an infection in someone microbes. These cells go into action each time who has the disease. But there are three infecting microbes enter the body. If they are limitations of this approach to dealing with successful, we do not actually come down infectious disease. The first is that once with any disease. The immune cells manage someone has a disease, their body functions to kill off the infection long before it assumes are damaged and may never recover major proportions. As we noted earlier, if the completely. The second is that treatment will number of the infecting microbes is take time, which means that someone controlled, the manifestations of disease will suffering from a disease is likely to be be minor. In other words, becoming exposed bedridden for some time even if we can give to or infected with an infectious microbe does proper treatment. The third is that the person not necessarily mean developing noticeable suffering from an infectious disease can serve disease. as the source from where the infection may spread to other people. This leads to the So, one way of looking at severe infectious multiplication of the above difficulties. It is diseases is that it represents a lack of success because of such reasons that prevention of of the immune system. The functioning of the diseases is better than their cure. immune system, like any other system in our body, will not be good if proper and sufficient How can we prevent diseases? There are nourishment and food is not available. two ways, one general and one specific to each Therefore, the second basic principle of disease. The general ways of preventing prevention of infectious disease is the infections mostly relate to preventing availability of proper and sufficient food for exposure. How can we prevent exposure to everyone. infectious microbes? If we look at the means of their spreading, WHY DO WE FALL ILL 185

Activity _____________ 13.6 immunisation. Immunisation • Conduct a survey in your locality. Talk to ten families who are well-off Traditional Indian and Chinese medicinal and ten who are very poor (in your systems sometimes deliberately rubbed the estimation). Both sets of families skin crusts from smallpox victims into the should have children who are below skin of healthy people. They thus hoped five years of age. Measure the heights to induce a mild form of smallpox that of these children. Draw a graph of the would create resistance against the height of each child against its age disease. for both sets of families. Famously, two centuries ago, an • Is there a difference between the English physician groups? If yes, why? named Edward Jenner, realised • If there is no difference, do you think that milkmaids that your findings mean that being who had had well-off or poor does not matter for cowpox did not health? catch smallpox even during These are the general ways of preventing epidemics. infections. What are the specific ways? They Cowpox is a very relate to a peculiar property of the immune mild disease. system that usually fights off microbial Jenner tried infections. Let us cite an example to try and deliberately giving understand this property. cowpox to people These days, there is no smallpox (as he can be seen doing in the picture), and anywhere in the world. But as recently as a found that they were now resistant to hundred years ago, smallpox epidemics were smallpox. This was because the smallpox not at all uncommon. In such an epidemic, virus is closely related to the cowpox virus. people used to be very afraid of coming near ‘Cow’ is ‘vacca’ in Latin, and cowpox is someone suffering from the disease since they ‘vaccinia’. From these roots, the word were afraid of catching the disease. ‘vaccination’ has come into our usage. However, there was one group of people who did not have this fear. These people would We can now see that, as a general principle, provide nursing care for the victims of we can ‘fool’ the immune system into smallpox. This was a group of people who had developing a memory for a particular infection had smallpox earlier and survived it, although by putting something, that mimics the microbe with a lot of scarring. In other words, if you we want to vaccinate against, into the body. had smallpox once, there was no chance of This does not actually cause the disease but suffering from it again. So, having the disease this would prevent any subsequent exposure once was a means of preventing subsequent to the infecting microbe from turning into attacks of the same disease. actual disease. This happens because when the immune Many such vaccines are now available for system first sees an infectious microbe, it preventing a whole range of infectious responds against it and then remembers it diseases, and provide a disease-specific specifically. So the next time that particular means of prevention. There are vaccines microbe, or its close relatives enter the body, against tetanus, diphtheria, whooping cough, the immune system responds with even measles, polio and many others. These form greater vigour. This eliminates the infection the public health programme of childhood even more quickly than the first time around. This is the basis of the principle of 186 SCIENCE

immunisation for preventing infectious humans and animals. Find out the diseases. plan of your local authority for Of course, such a programme can be Q uestionsthe control of rabies in your useful only if such health measures are neighbourhood. Are these measures available to all children. Can you think of adequate? If not, what improvements reasons why this should be so? would you suggest? Some hepatitis viruses, which cause 1. Why are we normally advised to jaundice, are transmitted through water. take bland and nourishing food There is a vaccine for one of them, hepatitis when we are sick? A, in the market. But the majority of children in many parts of India are already immune 2. What are the different means by to hepatitis A by the time they are five years which infectious diseases are old. This is because they are exposed to the spread? virus through water. Under these circumstances, would you take the vaccine? 3. What precautions can you take in your school to reduce the Activity _____________ 13.7 incidence of infectious diseases? • Rabies virus is spread by the bite of 4. What is immunisation? infected dogs and other animals. There 5. What are the immunisation are anti-rabies vaccines for both programmes available at the nearest health centre in your locality? Which of these diseases are the major health problems in your area? What you have learnt • Health is a state of physical, mental and social well-being. • The health of an individual is dependent on his/her physical surroundings and his/her economic status. • Diseases are classified as acute or chronic, depending on their duration. • Disease may be due to infectious or non-infectious causes. • Infectious agents belong to different categories of organisms and may be unicellular and microscopic or multicellular. • The category to which a disease-causing organism belongs decides the type of treatment. • Infectious agents are spread through air, water, physical contact or vectors. • Prevention of disease is more desirable than its successful treatment. • Infectious diseases can be prevented by public health hygiene WHY DO WE FALL ILL 187

measures that reduce exposure to infectious agents. • Infectious diseases can also be prevented by using immunisation. • Effective prevention of infectious diseases in the community requires that everyone should have access to public hygiene and immunisation. Exercises 1. How many times did you fall ill in the last one year? What were the illnesses? (a) Think of one change you could make in your habits in order to avoid any of/most of the above illnesses. (b) Think of one change you would wish for in your surroundings in order to avoid any of/most of the above illnesses. 2. A doctor/nurse/health-worker is exposed to more sick people than others in the community. Find out how she/he avoids getting sick herself/himself. 3. Conduct a survey in your neighbourhood to find out what the three most common diseases are. Suggest three steps that could be taken by your local authorities to bring down the incidence of these diseases. 4. A baby is not able to tell her/his caretakers that she/he is sick. What would help us to find out (a) that the baby is sick? (b) what is the sickness? 5. Under which of the following conditions is a person most likely to fall sick? (a) when she is recovering from malaria. (b) when she has recovered from malaria and is taking care of someone suffering from chicken-pox. (c) when she is on a four-day fast after recovering from malaria and is taking care of someone suffering from chicken-pox. Why? 6. Under which of the following conditions are you most likely to fall sick? (a) when you are taking examinations. (b) when you have travelled by bus and train for two days. (c) when your friend is suffering from measles. Why? 188 SCIENCE