Flu shot

Also known as: Flu vaccine, Influenza vaccine, Influenza shot
The flu shot is a vaccine that contains a killed form of three flu viruses. The vaccine causes your immune system to make antibodies. If you are exposed to the flu later, the antibodies can attack and destroy the virus. 

It's most important to get a flu shot if you're at high risk for other health problems from the flu. Those at high risk include young children, pregnant women, older adults, and people with chronic diseases or weak immune systems.