20 Myths and facts about cold and flu

The flu vaccine causes the flu

Why it's not true: The viruses contained in flu shots have been killed (the pros say they been "inactivated"), which means they really can't cause infection. Getting a flu shot may cause symptoms that feel like the flu, though, admits Susan Coffin, M.D., medical director of infection prevention and control at The Children's Hospital of Philadelphia. "Most often, people misattribute the achy side effect that can sometimes follow the flu shot with the onset of the flu itself," she says. "There are also so many other respiratory viruses floating around out there that you could catch one after you get the flu vaccine, and it may seem as though the shot caused it. It hasn't."