About a hundred years ago,when smallpox persisted,a group of people who had this disease earlier and had survived it had no chance of suffering from it again. So,having the disease once was a means of preventing subsequent attacks of the same disease.
$(a)$ Why does this happen? Explain.
$(b)$ We can 'fool' the immune system. Clarify.
$(c)$ Name this principle of prevention of diseases.

(C) This happens because when the immune system first encounters an infectious microbe,it responds against it and then remembers it specifically. So,the next time that particular microbe,or its close relatives,enters the body,the immune system responds with even greater vigour. This eliminates the infection even more quickly than the first time. This is the basis of the principle of immunisation.
$(b)$ We can 'fool' the immune system into developing a memory for a particular infection by introducing something into the body that mimics the microbe we want to vaccinate against. This does not actually cause the disease,but it prevents any subsequent exposure to the actual infecting microbe from turning into a disease.
$(c)$ This principle of disease prevention is called immunisation,which involves providing strong immunity to the body through vaccines. Many such vaccines are now available for preventing a wide range of infectious diseases and provide a disease-specific means of prevention. Examples include vaccines against tetanus,diphtheria,whooping cough,measles,and polio.