The annual flu season hit the United States about a month early this year, and illness is now widespread in 47 states.Many cases are caused by a flu strain that tends to make people sicker.
But so far, experts say it's too early to know whether this will end up being a bad season.
There are signs the flu may have already peaked in a few states, though it's too early to tell for sure, health officials say.
This season's vaccine is well-matched to the circulating strains, and there's still some available.
The vaccine is 62 per cent effective, according to government study results released last Friday.
Health officials are urging people to get vaccinated; it's recommended for everyone six months or older.
Besides getting a flu shot, Americans have been urged to wash hands with soap and warm water, and avoid touching their eyes, nose and mouth, and to keep away from sick people.