Over the last century, vaccination has been the most effective medical strategy to control infectious diseases. Explain the principle on which 'vaccinations' act.

The principle of vaccination is based on the property of "memory" of the immune system. A preparation of antigenic proteins or pathogen or inactivated / weakened pathogen are introduced into the body. The antibodies produced in the body against these antigens would neutralize the pathogenic agents during actual infection. Memory T cell and B cell that recognize the pathogen quickly on subsequent exposure and overwhelm the invaders with a massive production of antibodies.