Answer: Vaccine
A vaccine is a biological preparation that contains the weakened or the killed microorganisms like viruses or the bacteria that is given in the form of an injection in order to increase the protection against the particular disease.
It is like a training course for the immune system. They prepare the body to fight against the disease. Whenever, the body is exposed to foreign invaders like bacteria and viruses, the body immune cells called as lymphocytes act against them by producing antibodies against them.
Example: MMR (measles, mumps, and rubella) vaccine, chickenpox vaccine etc.