Vaccination involves the administration of a killed microbe to trigger the body's defense mechanism against that microbe, thereby preventing illness. Immunizations function by activating the immune system, the body's natural defense against diseases. A robust immune system can identify invading bacteria and viruses, producing antibodies to eliminate or neutralize them. Immunizations equip the immune system to fend off diseases. For viral diseases, vaccines contain viruses that have been either weakened or killed. In contrast, for bacterial diseases, vaccines often utilize a small portion of the dead bacteria to prompt the immune system to generate antibodies against the entire bacteria. Furthermore, the efficacy of immunizations can be enhanced through periodic additional injections, known as "boosters." For more information, see the plural term "immunizations" and specific types of vaccines.