The Centers For Disease Control is warning that the flu is spreading faster and earlier than usual this season.
The CDC says there is widespread flu activity in 43 states.
California has, so far, been spared an outbreak, but doctors say it's only a matter of time before the flu is widespread in the state.
According to doctors, even people who have received the flu shot can still get sick.
Experts also say there are a lot of flu viruses out there, and the vaccine only protects against three of them.