CBSE Class 9 Answered

Hi Shyam,

Immunisation is the process by which an individual's immune system is equipped to fight off infectious agents.

The principle of immunisation is suffering from a disease is a means of preventing subsequent attacks of the same disease. This is because when the immune system first encounters an infectious agent, it responds to it and remembers the infectious agent specifically. So the next time when the particular microbe or its close relative enters the body, the immune systems responds with greater intensity. This eliminates the infection more quickly than the first time.