How do vaccinations work?

A vaccination works by giving you a very small dose of a bacterium, virus, or toxin to prepare your immune system to quickly mobilize and fight in the event you are exposed to it in the future.

Tiny organisms (like viruses and bacteria) can attack your body and cause infections that make you sick. When you get an infection, your body makes special disease-fighting substances called antibodies to fight the organism. In many cases, once your body has made antibodies against an organism, you become immune to the infection it causes. Immune means you are protected against getting an infection. If you're immune to an infection, it means you can't get the infection.

Vaccines usually have a small amount or piece of the organism that causes an infection. The organisms used in vaccines are generally weakened or killed so they won’t make you sick. The vaccine causes your body to make antibodies against the organism. This allows you to become immune to an infection without getting sick first.

Some vaccines have a live but weakened organism. These are called live-virus vaccines. While live-virus vaccines are usually safe for most babies and adults, they’re not generally recommended for pregnant women.