Vaccine (or immunization) is the process of preparing your body to fight off infectious diseases like measles, tetanus, rubella, whooping cough and polio. Vaccines work by introducing a weakened version of a disease-causing germ into your body, which prompts your immune system to develop antibodies against the pathogen. Vaccines are the silent heroes of public health, protecting millions of people each year from illnesses that used to be commonplace, and helping eradicate deadly pathogens like smallpox.

In order to make a vaccine, scientists first identify the part of the virus or bacteria that triggers the immune response, which is then incorporated into the vaccine formulation. In some cases, the vaccine is made up of whole killed pathogens, while in others it contains just a weakened form of the pathogen or even just the chemical structure for one of its proteins. In the future, if your body encounters the same pathogen again, the antibodies it created will instantly recognize and destroy it, much as it would a live, dangerous virus or bacteria.

Vaccines also cause your memory cells to create a kind of wanted poster that shows the pathogen, so you’re more likely to remember and quickly respond to it in the future. Once enough people in a community get vaccinated against a particular illness, it becomes much less likely to spread from person to person. This is known as “herd immunity.”