Norovirus cases are spiking this winter. Here's why — and what you need to know to protect yourself.

• Norovirus cases are surging in the US to some of the highest levels in more than a decade.

• More people are gathering post-pandemic, contributing to a rise in cases from past years.

• Staying hydrated is the most important thing to do if you become sick.

Norovirus, commonly called the stomach flu or the 24-hour bug, is on the rise across the US this winter.

Data from the Centers for Disease Control and Prevention indicates that reports of norovirus outbreaks — which are most common from November to April — reached some of their highest levels of the past 12 years.

The CDC reported 91 norovirus outbreaks across the US for the first week of December, the latest data available. That was the most for that week since 2012 and the fourth-most of any week since 2012.

The CDC defines an outbreak as "an occurrence of two or more similar illnesses resulting from a common exposure that is either suspected or laboratory-confirmed to be caused by norovirus."

Here's what you need to know about norovirus.

Norovirus symptoms, treatment, and prevention

Norovirus presents much like food poisoning — with a rapid onset of nausea, diarrhea, vomiting, and stomach pain that generally lasts one to three days. But there are a few key differences.

How it spreads: Once someone is exposed to norovirus — which is spread through direct contact and can linger on surfaces for days or even weeks — they can become infected within a day or two.

The symptoms of food poisoning, on the other hand, will typically present much faster than that, in a matter of hours, said Dr. Shruti Gohil, a professor and the associate medical director of epidemiology and infection prevention at the University of California, Irvine's School of Medicine.

How to treat: Though the treatments for norovirus and food poisoning may be similar — drinking water, eating bland foods, taking antidiarrheal medication — Gohil said that when in doubt, act as if you have norovirus.

That's because of how highly contagious norovirus is compared with food poisoning. Norovirus is one of the most contagious infections we know of, and to help prevent its spread, you need to be vigilant about washing your hands and disinfecting surfaces, she said.

The most important thing to do if you're experiencing symptoms is to stay hydrated. When you're losing so much fluid, particularly if you can't keep any fluids down, there's a risk of dehydration, which can become life-threatening, Gohil said.

Look for these symptoms as warning signs: If you become dizzy or lightheaded, Gohil said, then you may be dangerously dehydrated and should get yourself to an emergency room.

She added that a headache is an early warning sign of dehydration and an indicator that you need to increase your fluid intake.

People who are very young, very old, or immunocompromised can have infections that last longer, and they may be at higher risk of dehydration.

Pay special attention: Gohil also mentioned a few signs of dehydration to look out for in people who can't speak, like babies and nonverbal older people: they can't form tears, their mouth looks dry, they're despondent, or they're not really reacting to you.

Why norovirus is so bad this year

Gohil said the total number of norovirus outbreaks was pretty bad this year.

One of the main reasons for that, she said, is that more people gathered for the holiday season this year than they did in the past few years when COVID-19 was a more immediate concern.

"We've got more outbreaks, people are unmasked, people are engaging with each other, they're not cleaning their hands, clearly," Gohil said. "Cleaning and disinfection and all that stuff is down. And we need to shore those practices back up."

Gohil said that before the pandemic there were years when norovirus outbreaks spiked just as they are now and that fluctuations like this are normal.

She said everyone has an important role to play in helping prevent the spread of norovirus.

"You could literally, just by being careful yourself, save other people's lives indirectly. That's really, really important for people to understand, that you live in a community, " she said, "and that you have a responsibility to care, and to keep yourself clean, and keep others safe."

In other words, wash your hands.

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