Classically, vaccines are made from real viruses. One way is to weaken them to the point that they no longer cause illness but can still stir up an antibody response; that is how Louis Pasteur, one of the founding figures of microbiology, created a vaccine for cholera in chickens. Chemically inactivated viruses can fool the body into believing it is being infected; such vaccines have been used for encephalitis and rabies.
