A vaccine is medicine that trains your body to fight foreign invasions, and in many cases, provides acquired immunity to the invader. All vaccines begin with an exposure to a virus or pathogen to trigger a strong cellular immune response that involves your MHC class I natural killer T cells. They learn to respond to that viral infection and eliminate it.

Vaccines have been with us for more than 200 years and no advance in public health short of sanitation has been more important. Both control infections, the leading causes of death in the world.

The origin of the word vaccine comes from vacca, the Latin word for cow. That’s because the vaccination story begins with Edward Jenner, an English country doctor who used a live but weakened or attenuated foreign antigen found in the exudate (pus) from a pustule of cowpox and injected it into a boy to create a state of immune protection against the smallpox virus.

The inoculation worked. In 1880, Louis Pasteur built on the work of Jenner when he realized old chicken cholera germs could not transmit the disease and used them to develop a lab-created vaccine to inoculate chickens against the disease. He later created animal vaccines against rabies and anthrax.

Jenner’s and Pasteur’s monumental work using weakened or attenuated organisms to create a state of immune protection is the foundation for the live-attenuated vaccines we are familiar with today.

The principles of this vaccination depend on using these organisms—or, more recently, parts of the organism—to produce an immune response. Adaptive and cellular immunity to specific infections can be achieved by deliberately causing that infection with an unmodified infectious agent like Jenner did with smallpox.