Class IX: Biology
Chapter 4 : Why do we fall ill
1) Our body’s well-being is dependent on the proper functioning of its cells and tissues.
2) All our body parts and activities are greatly interconnected. Hence dysfunction of any body part will affect the entire body.
3) When we are healthy, we are able to perform our physical, mental and social functions well.
4) Our physical and social environment plays an important role in maintaining good health.
5) In addition to personal hygiene, public cleanliness should also be maintained for ensuring that we remain healthy.
6) Many other factors like financial conditions, availability of nutritious food and social equality also influence the health of an individual.
7) A person suffering from a disease is in a state of discomfort.
8) Being in poor health is different from being diseased.
9) Symptoms and signs of the disease appear as a result of dysfunction of the affected body parts. These help to identify the disease that a person is
10) Depending on their duration, diseases may be classified as acute or chronic.
11) Acute diseases last for a short time and do not cause major health effects.
E.g. Common cold. On the other hand chronic diseases persist for a long time and hence cause prolonged ill health. E.g. TB of lungs.
12) Causes of diseases may be immediate (first level cause) or contributory. For
e.g. – The immediate cause of a person suffering from diarrhoea is the causative agent. The contributory causes could be
i) lack of good nourishment or genetic difference (second level cause),
ii) poverty or lack of public services (third level cause)
13) Diseases may be infectious or non-infectious. Infectious diseases are caused by microbes or other infectious agents (e.g. Malaria) whereas non-infectious diseases have internal, non-infectious causes (e.g. High blood pressure).
14) The infectious agents may be viruses, bacteria, fungi, protozoans or multicellular organisms like worms.
Category of infectious agent Examples of diseases caused
Common cold, influenza, Dengue fever, AIDS
Bacteria Typhoid, cholera, tuberculosis, anthrax
Protozoans Malaria, kala-azar
Worms Intestinal worm infections, elephantiasis
15) The type of treatment of a disease depends on the category of the infectious agent.
16) Organisms belonging to one category will share several important life processes, which will be different from those of organisms belonging to other
17) Drugs working by interfering with life processes of one category of organisms will not be effective against members of another category. E.g. Antibiotics act against bacteria, but not viruses, since viruses do not share bacterial pathways.
18) Penicillin interferes with bacterial cell wall production, thus killing the
bacteria. Due to lack of cell walls, penicillin does not affect human cells.
19) Many infectious diseases are called ‘communicable diseases’ since they can spread from one person to another.
20) Communicable diseases can spread through air, water, food, sexual contact or vectors.
21) The droplets released during coughing or sneezing of an infected person causes the spread of air-borne diseases like common cold, pneumonia,
22) Air-borne diseases spread quickly in overcrowded and poorly ventilated living conditions.
23) Water-borne diseases like cholera spread when drinking water gets
contaminated with the infectious agents.
24) Sexual contact causes the spread of diseases like AIDS and syphilis from the infected person to a healthy one. AIDS virus can also spread through blood transfusions, use of infected needles or during pregnancy and breast-feeding
by an infected mother.
25) Vectors are intermediate animals causing the spread of disease causing agents from an infected person to a healthy person. For example – Female
mosquitoes transmit many diseases like malaria when they feed on the blood of animals and humans.
26) On entering the body, the infectious agents reach their specific target
organs. In certain cases the target organ is related to their point of entry. For
e.g. – Typhoid causing bacteria enter through mouth and reside in gut lining.
27) In other instances, the target organ of the microbe has no relation to their point of entry. For e.g. HIV enters through the sexual organs but spreads to
all the lymph nodes.
28) The symptoms of a disease depend on the target organ infected by the microbe. For e.g. cough and breathing problems are seen when lungs are
infected. So based on the signs and symptoms of a disease, we can get an idea of the target organ of the microbe.
29) During infection, the body’s activated immune system sends specialized cells to destroy the microbes, causing inflammation, with associated local effects.
30) The AIDS causing virus destroys the functioning of the immune system, due to which the body becomes unable to fight even minor infections. Ultimately the patient succumbs to such infections.
31) The severity of a disease is directly proportional to the number of infectious agents present in body.
32) An infectious disease can be treated in 2 ways:
i) Reduce the symptoms of the disease by providing treatment
ii) Kill the infectious agent causing the disease.
33) Medicines used in killing an infectious agent aim to disrupt some pathway of a vital life function peculiar to that group of organisms. These pathways are not present in other microbial groups or in humans.
34) On entering human cells, viruses use our cellular machinery for carrying out all their life processes. There are very few virus-specific biochemical
pathways that can be targeted to produce anti-viral drugs.
35) The approach of treatment of an infectious disease has three drawbacks:
i) Recovery of the patient may not be complete in certain cases.
ii) Treatment requires time; hence the patient suffers from the
disease and may be bedridden.
iii) The patient serves as the source of spread of infection to others.
36) It is desirable to prevent a disease than to treat it completely.
37) There are general and specific ways of preventing diseases.
38) Infectious diseases can be generally prevented by public health hygiene methods, which aim to reduce exposure to infectious microbes. Public hygiene measures include providing safe drinking water, clean environments and adequately spacious conditions for living.
39) Another general method of preventing infectious diseases requires the availability of sufficient and balanced diet for the proper functioning of the immune system. The immune system ensures that we do not develop a disease each time we are exposed to an infectious agent, by destroying the agent before it multiplies greatly.
40) During smallpox epidemics, it was noted that people who survived after suffering form smallpox, did not get infected with it again. Such observations led to the birth of immunization, which is a specific method of preventing infectious diseases.
41) The principle of immunization is based on the memory of the immune system on encountering an infectious agent. On subsequent encounters with the
same or related microbe, the response of the immune system is multiplied greatly, leading to quick elimination of the infection.
42) During immunization, a vaccine (containing weakened or killed pathogen or a specific part of the pathogen) is introduced into the body to fool the immune
system into remembering a particular infection. Hence the body does not suffer even on further exposures to that pathogen or its close relatives.
43) Nowadays, vaccines preventing many infectious diseases like tetanus, polio and measles are used extensively especially in child health immunization programmes.
44) Everyone in the community should have access to public hygiene and immunization for effective prevention of infectious diseases.