(NewsNation) — Concern about a “quad-demic” is growing in much of the country as the nation’s hospitals see a surge in respiratory viruses.

In Southern California, most hospitals never ended their mask mandates after the COVID-19 pandemic. But now, more mandates are returning.

Medical experts predicted the surge in cases due to the holiday season, with millions of Americans mixing and mingling at airports and family gatherings.

Now, further spread is expected as millions head back to work and school.

Hospitalizations are up sharply in major cities, including Los Angeles, New York and Chicago.

Health officials are advising the immunocompromised to mask in public to prevent illness, and in Wisconsin, a major health care group is reinstating a mask mandate beginning next week.

The “quad-demic” means a quadruple threat of flu, COVID-19, RSV and norovirus, more commonly known as the stomach flu.

Norovirus has been rampant in many communities, and for most, the stomach flu means a very unpleasant 48 hours.

“It hit very suddenly in the middle of the night. I had to go throw up,” said Theo Ash, who recovered from norovirus. “But then it would just keep coming up and coming back and coming back, and I couldn’t hold anything down. It did rip through a group of us. I got it from my girlfriend, who got it from work.”

There is no vaccine for norovirus, but there are options for other illnesses. However, vaccination rates are down across the board due to what some have called a COVID-19 hangover, and that’s contributing to the surge in illness.

“It’s harder and harder to talk about vaccines with people. We’re just sick of hearing about respiratory viruses, but the reality is they’re still here. You don’t realize it’s going to hurt until it really does and so I would just encourage people that it’s not too late,” said Dr. Andrew Jameson.

Exposure and hygiene are key in preventing all four illnesses, including hand washing and staying home when ill.

However, while more hospital mandates are being implemented, masking in public places still remains a personal choice.