Experts are warning this will be a severe flu season, so visitors and patients have been urged to wear a mask at Sentara Obici Hospital and Sentara’s freestanding BelleHarbour emergency department, as well as at Bon Secours Health Center at Harbour View.

The “strong recommendation” also became effective Wednesday at more than two dozen other health care facilities across Hampton Roads. Hospitals and health care systems made the joint call in the face of mounting flu cases and a vaccine that’s significantly less effective than in the past.

Bon Secours emergency department medical director Carl Wentzel said the facility at Harbour View began seeing patients testing positive for influenza shortly after Thanksgiving.

“In the past 10 days to two weeks, we’ve started seeing a lot of it,” he said Tuesday. “The curve is headed upwards right now.”

Joel Michael, an attending emergency room physician at Sentara Obici Hospital, said that in 2013 and other recent prior years, influenza didn’t arrive until January or so.

This year, Michael has also seen it hit already. “In my personal practice, I have noticed it in the past two to three weeks,” he said.

Public health officials say seasonal influenza is “widespread” — the highest level — in Virginia and North Carolina.

Experts predict a severe season after the genetic makeup of the main circulating strain “drifted.” That means this year’s vaccine no longer matches the prevalent strain.

When developing the vaccine, officials try to predict what next year’s flu is going to look like, Wentzel said.

“They don’t know the exact genetics,” he said. “The past several years they pretty much hit the mark,” but didn’t come as close this year.

Even so, Wentzel and Michael strongly urge people to get flu shots this year. “The majority of the people I’m seeing did not get a flu shot,” Wentzel said.

“If you have not gotten a flu shot, you’ve still got time and it’s still important. Even if you do happen to get a strain not covered, it’s still helpful — it makes the illness less severe.”

Wentzel cited patients with temperatures as high as 104 degrees. Onset is rapid this year, he said — patients are feverish within 24 hours of beginning to feel bad.

Other symptoms include head and muscle aches, joint pain, sore throat and lethargy, and in some cases, nausea and even vomiting.

To avoid getting the flu, stay away from people with the flu, Michael said. Flu sufferers, he said, should stay at home — and in a different part of the house than other household members.

High-risk flu patients — children under 2, adults over 65, anyone with chronic lung disease or a disease-compromised immune system, like HIV sufferers or cancer patients undergoing chemotherapy — should discuss antiviral drugs with their health care provider, he said.

Finally, Michael said, hand washing is a must.

People with flu-like symptoms are also being asked not to visit patients at area hospitals.