$(a)$ Who discovered the first vaccine?
$(b)$ Name two viral diseases that can be prevented by using vaccines. What is immunity?

(N/A) Edward Jenner discovered the first vaccine.
$(b)$ Two viral diseases that can be prevented by vaccines are Hepatitis $A$ and Polio.
Immunity is the ability of an organism to resist a particular infection or toxin by the action of specific antibodies or sensitized white blood cells. It is the body's defense mechanism against disease-causing microorganisms (antigens). When antigens enter the body,the immune system produces antibodies to neutralize them,thereby preventing the disease.