CBSE Class 9 Answered

(i)Vaccines work by stimulating our immune system to produce antibodies (substances produced by the body to fight disease) without actually infecting us with the disease.

(ii)Vaccines trigger the immune system to produce its own antibodies against disease, as though the body has been infected with it. This is called "active immunity".

(iii)If the vaccinated person then comes into contact with the disease itself, their immune system will recognise it and immediately produce the antibodies they need to fight it.