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A vaccine is a preparation that contains weakened, dead or artificial antigens that when injected into the body stimulate the immune system to produce antibodies that can kill the real disease carrying antigens of the same type. Since the injected antigens are not capable to start an infection, vaccines are by and large safe.

Additionally, the antibodies produced after vaccination remain in the body providing protection from the antigen or to some degree from similar antigens for quite some time in the future depending on the vaccine.

A vaccine therefore provides increased resistance to infectious diseases. Each vaccine is designed to increase immunity against one or more similar types of antigens.