The story of vaccines did not begin with the first vaccine–Edward Jenner’s use of material from cowpox pustules to provide protection against smallpox. Rather, it begins with the long history of infectious disease in humans, and in particular, with early uses of smallpox material to provide immunity to that disease.
Evidence exists that the Chinese employed smallpox inoculation (or variolation, as such use of smallpox material was called) as early as 1000 CE. It was practiced in Africa and Turkey as well, before it spread to Europe and the Americas.
Edward Jenner’s innovations, begun with his successful 1796 use of cowpox material to create immunity to smallpox, quickly made the practice widespread. His method underwent medical and technological changes over the next 200 years, and eventually resulted in the eradication of smallpox. Today many diseases can be prevented with a vaccine.
How the vaccine works
Vaccines protect you by preparing your immune system to recognize and fight serious, and sometimes deadly, diseases. A vaccine contains a specific part of a germ (bacteria or virus), called an antigen. The antigen is killed or disabled before it's used to make the vaccine, so it can't make you sick. Vaccines, and the antigens they contain, stimulate your immune system's B cells to develop protective substances called antibodies. These antibodies are responsible for killing germs that enter your body. Once activated, B cells can stay in your body for a lifetime and allow your body to remember the germ that stimulated their creation. Throughout your life, these cells will recognize and fight the actual disease caused by the germ when and if you come into contact with it.Vaccines help prevent infectious diseases and save many lives. Because of vaccines, diseases including polio and measles, that once took devastating tolls on families and whole communities, are now almost extinct.
Source: New York Dept of Health