1. Chronic diseases have more drastic long-term effects on health compared to acute diseases. While acute diseases resolve quickly, chronic diseases persist for a long time or lifetime, reducing ability to function physically, mentally and socially over years. 2. Diseases can have multiple levels of causes, from immediate causes like infectious agents or viruses, to contributory causes like poor nutrition or lack of access to clean water. The immediate cause may be necessary but not sufficient on its own to cause disease without additional contributory social and environmental factors. 3. Causes of diseases can be infectious, spreading between individuals through microbes, or non-infectious, arising from internal factors like genetics rather than external transmission. The nature of

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

2. 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.
If health means a state of physical, mental Good economic conditions and jobs are
and social well-being, it cannot be something therefore needed for individual health.
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
But even more importantly, human beings connections between community issues and
live in societies. Our social environment, individual health.
therefore, is an important factor in our
individual health. We live in villages, towns 13.1.3 DISTINCTIONS BETWEEN ‘HEALTHY’
or cities. In such places, even our physical AND ‘DISEASE-FREE’
environment is decided by our social
environment. If this is what we mean by ‘health’, what do
Consider what would happen if no agency we mean by ‘disease’? The word is actually
is ensuring that garbage is collected and self-explanatory – we can think of it as
disposed. What would happen if no one takes ‘disease’ – disturbed ease. Disease, in other
responsibility for clearing the drains and words, literally means being uncomfortable.
ensuring that water does not collect in the However, the word is used in a more limited
streets or open spaces? meaning. We talk of disease when we can find
So, if there is a great deal of garbage a specific and particular cause for discomfort.
thrown in our streets, or if there is open drain- This does not mean that we have to know the
water lying stagnant around where we live, absolute final cause; we can say that
the possibility of poor health increases. someone is suffering from diarrhoea without
Therefore, public cleanliness is important for knowing exactly what has caused the loose
individual health. motions.
We can now easily see that it is possible
Activity _____________ 13.2 to be in poor health without actually suffering
from a particular disease. Simply not being
• Find out what provisions are made by diseased is not the same as being healthy.
your local authority (panchayat/ ‘Good health’ for a dancer may mean being
municipal corporation) for the supply
able to stretch his body into difficult but
of clean drinking water.
• Are all the people in your locality able
graceful positions. On the other hand, good
to access this? health for a musician may mean having enough
breathing capacity in his/her lungs to control
Activity _____________ 13.3 the notes from his/her flute. To have the
opportunity to realise the unique potential
• Find out how your local authority in all of us is also necessary for real health.
manages the solid waste generated in So, we can be in poor health without there
your neighbourhood.
being a simple cause in the form of an
• Are these measures adequate?
identifiable disease. This is the reason why,
• If not, what improvements would you
suggest? when we think about health, we think about
• What could your family do to reduce societies and communities. On the other
the amount of solid waste generated hand, when we think about disease, we think
during a day/week? about individual sufferers.
WHY DO WE FALL ILL 177

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

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

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

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

7. 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
categories of infectious agents? The answer • Find out how many of you in your class
is that these categories are important factors had cold/cough/fever recently.
in deciding what kind of treatment to use. • How long did the illness last?
• How many of you took antibiotics (ask
Members of each one of these groups –
your parents if you had antibiotics)?
viruses, bacteria, and so on – have many
• How long wer e those who took
biological characteristics in common.
antibiotics ill?
All viruses, for example, live inside host • How long were those who didn’t take
cells, whereas bacteria very rarely do. Viruses, antibiotics ill?
bacteria and fungi multiply very quickly, while • Is there a difference between these two
worms multiply very slowly in comparison. groups?
Taxonomically, all bacteria are closely related • If yes, why? If not, why not?
to each other than to viruses and vice versa.
This means that many important life
processes are similar in the bacteria group
13.3.2 MEANS OF SPREAD
but are not shared with the virus group. As a How do infectious diseases spread? Many
result, drugs that block one of these life microbial agents can commonly move from
processes in one member of the group is likely an affected person to someone else in a variety
to be effective against many other members of ways. In other words, they can be
of the group. But the same drug will not work ‘communicated’, and so are also called
against a microbe belonging to a different communicable diseases.
group. Such disease-causing microbes can
As an example, let us take antibiotics. spread through the air. This occurs through
They commonly block biochemical pathways the little droplets thrown out by an infected
important for bacteria. Many bacteria, for
person who sneezes or coughs. Someone
example, make a cell-wall to protect
standing close by can breathe in these
themselves. The antibiotic penicillin blocks
droplets, and the microbes get a chance to
the bacterial processes that build the cell-
wall. As a result, the growing bacteria become start a new infection. Examples of such
unable to make cell-walls, and die easily. diseases spread through the air are the
Human cells don’t make a cell-wall anyway, common cold, pneumonia and tuberculosis.
so penicillin cannot have such an effect on We all have had the experience of sitting
us. Penicillin will have this effect on any near someone suffering from a cold and
bacteria that use such processes for making catching it ourselves. Obviously, the more
cell-walls. Similarly, many antibiotics work crowded our living conditions are, the more
against many species of bacteria rather than likely it is that such airborne diseases will
simply working against one. spread.
182 SCIENCE

8. 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
water. This occurs if the excreta from someone
suffering from an infectious gut disease, such
as cholera, get mixed with the drinking water
used by people living nearby. The cholera-
causing microbes will enter new hosts
through the water they drink and cause
disease in them. Such diseases are much
more likely to spread in the absence of safe
supplies of drinking water. Fig. 13.3: Common methods of transmission of
The sexual act is one of the closest diseases.
physical contact two people can have with
each other. Not surprisingly, there are
microbial diseases such as syphilis or AIDS 13.3.3 O RGAN - SPECIFIC AND TISSUE -
that are transmitted by sexual contact from SPECIFIC MANIFESTATIONS
one partner to the other. However, such
sexually transmitted diseases are not spread The disease-causing microbes enter the body
by casual physical contact. Casual physical through these different means. Where do they
contacts include handshakes or hugs or go then? The body is very large when
sports, like wrestling, or by any of the other compared to the microbes. So there are many
ways in which we touch each other socially. possible places, organs or tissues, where they
Other than the sexual contact, the AIDS virus could go. Do all microbes go to the same tissue
can also spread through blood-to-blood or organ, or do they go to different ones?
contact with infected people or from an Different species of microbes seem to have
infected mother to her baby during pregnancy evolved to home in on different parts of the
or through breast feeding. body. In part, this selection is connected to
We live in an environment that is full of their point of entry. If they enter from the air
many other creatures apart from us. It is via the nose, they are likely to go to the lungs.
WHY DO WE FALL ILL 183

9. 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
thus depend on the tissue or organ which 13.3.4 PRINCIPLES OF TREATMENT
the microbe targets. If the lungs are the
targets, then symptoms will be cough and What are the steps taken by your family when
breathlessness. If the liver is targeted, there you fall sick? Have you ever thought why you
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
Based on what we have learnt so far, it
symptoms and signs of an infection will be if
would appear that there are two ways to treat
we know what the target tissue or organ is,
an infectious disease. One would be to reduce
and the functions that are carried out by this
the effects of the disease and the other to kill
tissue or organ.
the cause of the disease. For the first, we can
In addition to these tissue-specific effects
of infectious disease, there will be other provide treatment that will reduce the
common effects too. Most of these common symptoms. The symptoms are usually
effects depend on the fact that the body’s because of inflammation. For example, we can
immune system is activated in response to take medicines that bring down fever, reduce
infection. An active immune system recruits pain or loose motions. We can take bed rest so
many cells to the affected tissue to kill off the that we can conserve our energy. This will
disease-causing microbes. This recruitment enable us to have more of it available to focus
process is called inflammation. As a part of on healing.
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
In some cases, the tissue-specificity of the cured. For that, we need to be able to kill off
infection leads to very general-seeming the microbes.
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

10. 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
These pathways will not be used by us overcrowded. For water-borne microbes, we
either. For example, our cells may make new can prevent exposure by providing safe
substances by a mechanism different from drinking water. This can be done by treating
that used by bacteria. We have to find a drug the water to kill any microbial contamination.
that blocks the bacterial synthesis pathway For vector-borne infections, we can provide
without affecting our own. This is what is clean environments. This would not, for
achieved by the antibiotics that we are all example, allow mosquito breeding. In other
familiar with. Similarly, there are drugs that words, public hygiene is one basic key to the
kill protozoa such as the malarial parasite. prevention of infectious diseases.
One reason why making anti-viral In addition to these issues that relate to
medicines is harder than making anti- the environment, there are some other general
bacterial medicines is that viruses have few principles to prevent infectious diseases. To
biochemical mechanisms of their own. They appreciate those principles, let us ask a
enter our cells and use our machinery for question we have not looked at so far.
their life processes. This means that there Normally, we are faced with infections
are relatively few virus-specific targets to aim everyday. If someone is suffering from a cold
at. Despite this limitation, there are now and cough in the class, it is likely that the
effective anti-viral drugs, for example, the children sitting around will be exposed to the
drugs that keep HIV infection under control. infection. But all of them do not actually suffer
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
How can we prevent diseases? There are body, will not be good if proper and sufficient
two ways, one general and one specific to each nourishment and food is not available.
disease. The general ways of preventing Therefore, the second basic principle of
infections mostly relate to preventing prevention of infectious disease is the
exposure. How can we prevent exposure to availability of proper and sufficient food for
infectious microbes? everyone.
If we look at the means of their spreading,
WHY DO WE FALL ILL 185

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

12. immunisation for preventing infectious humans and animals. Find out the
diseases. plan of your local authority for
Of course, such a programme can be the control of rabies in your
Q
useful only if such health measures are neighbourhood. Are these measures
available to all children. Can you think of adequate? If not, what improvements
would you suggest?
reasons why this should be so?
Some hepatitis viruses, which cause
jaundice, are transmitted through water. uestions
There is a vaccine for one of them, hepatitis 1. Why are we normally advised to
A, in the market. But the majority of children take bland and nourishing food
in many parts of India are already immune when we are sick?
to hepatitis A by the time they are five years 2. What are the different means by
old. This is because they are exposed to the which infectious diseases are
virus through water. Under these spread?
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?
4. What is immunisation?
• Rabies virus is spread by the bite of
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

13. 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