It inevitably happens to people every flu season - they obediently get a flu shot only to catch the virus anyway.
But don't blame the flu shot.
Despite what many people believe, the flu shot doesn't cause the flu.
The influenza viruses in the shot are dead and can't cause infection, according to the Centers for Disease Control.
So why do people still get sick even after getting the shot?
One explanation is they could have a different strain of the virus than the one the vaccine will fend off. Drug makers create each season's vaccine based on which strain they think will be prominent.
It is also possible for people to be exposed to the virus before the shot was given or in the two weeks it takes the drug to start protecting the body.
Others may mistake their symptoms for the flu when they really have another respiratory illness or virus.
"I think people get sick with a cough, runny nose and body aches and automatically conclude they have the flu, but it could simply be another virus," saild Christina Hughes, a nurse and emergency preparedness coordinator at MedStar Franklin Square Medical Center.
The CDC also says that the vaccine works differently in everyone. The flu vaccine works best among young healthy adults and older children, the CDC said. Some older people and people with certain chronic illnesses might develop less immunity.
Health care professionals argue that the flu vaccine is still the best protection from the virus. As the country faces its worst flu season in a decade, they say there is still time for people to get the shot.
"The bottom line is the shot is recommended for people 6 months of age or older," Hughes said. "If you haven't gotten it you can still get it."