Your immune system is able to "recognize" pieces of proteins on the surface of a virus (its epitopes) as foreign. Your immune system also "remembers" the epitopes of virus strains that it has encountered in the past. This "remembering" of epitopes by the immune system is called specific immunity. It means that if a person has been exposed to a particular strain of virus in the past, he or she will respond more quickly in the future to a virus of the same strain by producing the same antibodies that were produced before.

A vaccine takes advantage of specific immunity by providing the antigens (containing the epitopes) of a virus in an illness-free way so that the immune system can recognize them (and later remember them) without any serious illness. A vaccine impersonates an active virus and helps the immune system prepare and respond faster when the active virus tries to infect later.

The very first vaccine was produced and administered in 1796 by Edward Jenner, who was trying to prevent smallpox, a deadly disease that was widespread at the time. Jenner made the vaccine out of material he took from the cowpox sores of a milkmaid because he had noticed that after a milkmaid got sick with cowpox, she never got smallpox. Jenner was observing specific immunity at work, as the cowpox virus was similar enough (epitope-wise) to prepare the milkmaid immune system for smallpox infection. Jenner called his preparation from cowpox sores a "vaccine" because "vacca" is the latin word for cow, and the success of his vaccine is reason we use the term vaccine today to describe any preparation of antigens designed to establish specific immunity to a disease.

A good vaccine can prevent a virus from causing an epidemic. If enough people get vaccinated, a virus can even be eradicated. Good luck with your vaccine!