A flu shot is a vaccine that protects against influenza, commonly known as the flu. It involves injecting a small amount of inactivated or weakened virus into the body to stimulate an immune response and help the body develop antibodies to fight off future infections with the actual virus. The shot is typically administered during flu season, which is usually October through March, but it can be given at any time of year. It's recommended for people who are at high risk of developing serious complications from the flu, as well as for those who come into close contact with them, such as healthcare workers and family members. The flu shot is not 100% effective in preventing the flu, but it can significantly reduce the severity and duration of illness if one does contract the virus.