Why do we fall ill
1) The droplets released during coughing or sneezing of an infected person causes the spread of air-borne diseases like common cold, pneumonia, tuberculosis etc.
2) Air-borne diseases spread quickly in overcrowded and poorly ventilated living conditions.
3) Water-borne diseases like cholera spread when drinking water gets contaminated with the infectious agents.
4) 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.
5) 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.
6) 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.
7) 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.
8) 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.
9) During infection, the body’s activated immune system sends specialized cells to destroy the microbes, causing inflammation, with associated local effects.
10) 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.
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NCERT 9 Science