In California, which is among the hardest-hit states, the virus struck surprisingly early this season. The state's warmer temperatures typically mean people are less confined indoors during the winter months. As a result, flu season usually strikes later than in other regions.
Health experts aren't sure why this season is different.
"We're seeing the worst of it right now," said Randy Bergen, a pediatrician who is leading the anti-flu effort for Kaiser Permanente Northern California. "We're really in historic territory, and I just don't know when it's going to stop."
Here are five things you should know about this flu season:
1. It's shaping up to be one of the worst in recent years.
The influenza A subtype H3N2 that appears to be most prevalent this year is particularly nasty, with more-severe symptoms including fever and body aches. Australia — whose experience U.S. public health officials follow closely in making their flu forecasts, in part because Australia's winter is our summer — reported a record-high number of confirmed flu cases in 2017. Another influenza B virus subtype also is circulating, "and that's no fun, either," Bergen said.
2. This season's flu vaccine is likely to be less effective than previous versions.
U.S. flu experts say they won't fully know how effective this season's vaccine is until the season is over. But Australia's experience suggests effectiveness was only about 10 percent. In the United States, it is 40 to 60 percent effective in an average season. Vaccines are less protective if strains are different than predicted and if unexpected mutations occur.
3. You should get the flu shot anyway.
Even if it is not a good match to the virus now circulating, the vaccine helps ease the severity and duration of symptoms if you come down with the flu.
High-dose vaccines are recommended for older people, who also are exceptionally vulnerable to illness, hospitalization and death related to the flu, according to the CDC.
"Some protection is better than no protection," Bergen said, "but it's certainly disappointing to have a vaccine that's just not as effective as we'd like it to be."
Shots may still be available from your doctor or local health clinic, as well as at some chain drugstores. Check the Vaccine Finder website at vaccinefinder.org for a location near you.
4. Basic precautions may spare you and your family from days in bed.
As much as possible, avoid people who are sick. Wash your hands frequently and avoid touching your mouth, nose and eyes.
Masks aren't particularly effective in keeping you from catching the flu, although they may help keep sick people who wear them from spreading their germs.
If you are sick, cover your cough and stay home from work if you can, Bergen said. Remaining hydrated, eating nutritious foods and exercising can also help strengthen your immune system.
Because elderly people are so vulnerable to the flu, some nursing homes and assisted-living facilities may limit visitors and resident activities, depending on the level of illness.
5. Don't mistake flu symptoms for those of a common cold.
The hallmarks of flu are fever and body aches that accompany cough and congestion, Bergen said.
If you feel as if you're having trouble breathing, or if your fever can't be controlled with medication such as Tylenol, check with your doctor. It's even more important for patients to see a doctor if they have a chronic medical condition such as diabetes or heart disease, or if they are young or elderly.
Kaiser Permanente doctors are now being advised to prescribe antiviral drugs such as Tamiflu — given as a pill or, for children, an oral suspension — even without a lab test for influenza, Bergen said. According to a report in the Los Angeles Times, however, Tamiflu supplies are running low.
And Bergen cautioned that these medications are only partly effective, reducing the length of illness by just a day or two.
Ostrov is a reporter for Kaiser Health News, a nonprofit health news service that is an editorially independent part of the Kaiser Family Foundation.