Biology - Why do We Fall Ill



  • Health means a state of mental, physical, and social well-being.

  • The health of an organism largely depends on his/her surroundings or the environment.

  • Major causes of poor health are - the garbage, which is thrown in an open area nearby residence or streets, or/and the open drain water lying stagnant around the residence area.

  • The public cleanliness is the key of good health.

  • Some diseases, last only for a short period of time, are known as acute diseases. E.g. cold, fever, etc.

  • The diseases that last for a long period of time, even as much as a lifetime, are known as chronic diseases. E.g. asthma, osteoporosis, etc.

  • Chronic diseases, normally, have very severe long-term effects on people’s health as compared to the acute diseases.

Infectious Diseases

  • When microbes are the immediate causes of a disease, it is known as infectious diseases.

  • Some of the major agents of infectious diseases are viruses, bacteria, fungi, and some single-celled animals (protozoans).

  • Some diseases are caused by the multicellular organisms; such as worms.

  • Kala-azar or black fever is caused by a protozoan parasite of genus Leishmania (shown in the image given below).

  • Acne is caused by staphylococci bacteria (shown in the image given below).

  • Sleeping sickness is caused by protozoan organism namely Trypanosoma (shown in the image given below).


Means of Spread

  • Most of the microbial agents can commonly move from an affected person to other in number of ways.

  • The microbial agents are ‘communicated,’ therefore, also known as communicable diseases.

Airborne Diseases

  • Some of the microbes can spread through the air; example of such airborne diseases are common cold, pneumonia, and tuberculosis.

AirTransmitted Diseases

Waterborne Diseases

  • Some diseases can also be spread through water, known as waterborne diseases. E.g. cholera etc.

Vector-borne Infections

  • Some diseases are transmitted by different animals including human beings; in fact, these animals carry the infecting agents. Therefore, such animals are intermediaries and known as ‘vectors’.

  • Mosquitoes are the most common vectors.


  • Infectious diseases can be prevented by means of public health hygiene measures.

  • Infectious diseases can be prevented through proper immunization (in advance).