why does a person who suffered once from small pox not suffer with it again ?
At the first time on getting infected by small pox virus, the body of the person does not contain any antibodies to fight against it. But as we first get infected, our body produces antibodies to fight against the small pox infection which remains in the body for lifetime. So, the next time when the person gets infected from small pox, the antibodies present in the body fight against this small pox viral infection; hence the person would not be affected by the disease. Thus, the person can suffer from small pox only once in his complete lifetime.