Active immunity is the resistance developed by an individual in response to an antigenic stimulus.
The antigen may gain entrance either by natural infection or through any other sources such as artificial immunization by vaccination.
a) Natural Active Immunity:smallpox
Immunity is developed by the host in response to the antigen that enters by natural infections.
For example, a person attacked by measles or smallpox develops natural active immunity as he recovers from the diseases.
The immunity acquired by way of such infections is also long lasting in many cases. For example, life time immunity is got following certain viral infections such as smallpox, measles and mumps.