A vaccine essentially helps your immune system crack the code of a certain illness. A vaccine is usually made of the same cells that could make you sick, but they are weak or inactive. Sometimes a vaccine is made of cells that are very close, but not exactly the same, to the cells that would make you sick.

When a vaccine enters the body, the immune system responds the same way it would to any germ. The vaccine is easier to fight than the illness you’re being vaccinated against, and it won’t make you sick while your immune system fights it. Once the immune system figures out how to fight and defeat the antigens, it remembers what works against them. Should such an enemy enter your body again, your body will move to attack it before it has a chance to implement its plans to make you sick. Sometimes, your immune system needs a refresher course, which is why you get booster doses of some vaccines. Some antigens are especially tricky, and change over time, like flu viruses. That’s why people need to get flu shots every year to make sure they’re ready to take on the latest version.