Why is it unusual to get the same infectious disease more than once?

Explanation

It is due to property of immune system called Memory. When the immune system encounters a specific foreign agent (e. g., a microbe) for the first time, generates immune response and eliminates the invader. This is called first encounter. The immune system retains the memory of the first encounter. As a result, a second encounter occurs more quickly and abundantly than the first encounter. It is called as anamnestic or booster response, which protects us from the same infectious agent or disease in future.