Question

Why chances of infection is very rare who were already infected by small pox?​

Answer 1

This is due to active immunity.
Explanation:
1. When small pox occurs body forms
antibodies to fight the infection
2. When subsequent encounter occur
with the same pathogen, antibodies
present in the body recognise the
pathogen.
3. And start destroying the pathogen
and thus the pathogen does not
harm the body in second encounter
and so on.
4. This type of immunity is long lasting
and has no side effects.