They work indirectly, by stimulating the body s immune system to produce antibodies. The human body responds to different vaccines by making different kinds of antibodies. For example, measles vaccine causes the body to make anti-measles antibodies. These antibodies then circulate throughout the body, on surveillance patrol, on the watch for germ invaders. These antibodies neutralize measles virus or tetanus toxin or other microbes that they encounter in the future.
Vaccines give the body time to prepare defenses against harmful invading germs.
In 1900, smallpox, diphtheria, measles, and other infections were leading causes of death in the United States. Our grandparents feared them. Today, these diseases are distant memories, due to the success of vaccines. If we stop vaccinating people, the number of people who are susceptible will gradually increase. When a large enough group is vulnerable to a contagious disease, outbreaks or epidemics can occur.