Question

If you had smallpox once, there was no chance of suffering from it again. Why does it happen ?

Answer 1

If you had smallpox once,there was no chance of suffering from it again because when the immune system first see an infectious microbe ,it responds against it and then remembers it specifically.So the next time thar particular microbe, or its close relatives enter the body,the immune system responds with even greater vigour.This eliminates the infection even more quickly than the first time around.This is the basis principal of immunisation.