1. The administration of antigenic material to produce immunity to a disease, which will prevent or ameliorate the effects of infection by a pathogen.
2. Injection of a dead, or inactivated, microbe in order to stimulate the immune system against the microbe, thereby preventing disease.

Vaccinations, or immunizations, work by stimulating the immune system, the natural disease-fighting system of the body.

The healthy immune system is able to recognize invading bacteria and viruses and produce substances (antibodies) to destroy or disable them.

Immunizations prepare the immune system to ward off a disease. To immunize against viral diseases, the virus used in the vaccine has been weakened or killed.

To only immunize against bacterial diseases, it is generally possible to use a small portion of the dead bacteria to stimulate the formation of antibodies against the whole bacteria.

In addition to the initial immunization process, it has been found that the effectiveness of immunizations can be improved by periodic repeat injections or "boosters".