A nasty stomach bug has been spreading across the country, including California, in recent months — leaving people feeling nauseous, achy or worse.

There were 91 outbreaks of norovirus reported across the country during the week of Dec. 5, up from 69 outbreaks the final week of November, according to the most recent numbers from the U.S. Centers for Disease Control and Prevention.

The CDC defines an outbreak as the occurrence of two or more similar illnesses resulting from a common exposure that is either suspected or laboratory-confirmed.

How does norovirus spread? What are common symptoms? And how can you keep from getting sick?

Here’s what to know:

What is norovirus?

Norovirus is a “very contagious” virus that causes acute gastroenteritis, an inflammation of the stomach or intestines, according to the CDC.

Although it’s sometimes called the “stomach flu” or “stomach bug,” norovirus is not related to the flu, which is caused by the influenza virus, the agency said.

Norovirus is the “leading cause of vomiting and diarrhea and food-borne illness in the United States,” with more than 2,500 norovirus outbreaks reported every year, according to the CDC.

People can get sick with norovirus year round, although outbreaks are most common from November to April, according to the CDC.

Peak norovirus season usually runs from December to March.

On average, norovirus causes 19 to 21 million illnesses in the U.S. each year, the CDC said.

“Children younger than 5 years old, older adults and people with weakened immune systems are more likely to develop severe infections,” the agency said on its website.

How does stomach flu spread?

Norovirus spreads easily and can make anyone ill, with symptoms usually lasting one to three days.

The virus most often spreads in closed and crowded environments, such as hospitals, nursing homes, child care centers and schools, according to the California Department of Public Health.

You can get norovirus in a variety of ways, federal and state health officials said, including:

•Eating or drinking something that has norovirus in it, including undercooked shellfish

•Touching a contaminated surface and then touching your mouth before washing your hands

•Having direct contact with someone with norovirus

•Sharing the toilet or bathroom with someone who is sick with norovirus

•Accidentally swallowing water that has norovirus in it, including water in swimming pools

•Cleaning up vomit or diarrhea without wearing protective gear, such as gloves and a face mask

While norovirus is closely associated with cruise ships, outbreaks account for only 1% of all reported norovirus cases, the CDC said.

In fact, acute gastrointestinal illness is relatively rare on cruise ships, the agency said.

What are common symptoms of norovirus?

People typically develop symptoms 12 to 48 hours after being exposed to norovirus, the CDC said.

According to the agency, the most common symptoms of the virus are:

•Diarrhea

•Vomiting

•Nausea

•Stomach pain

•Fever

•Headache

•Body aches

•Dehydration

•Decreased urination

•Dry mouth and throat

Can I die from norovirus?

Although most people will usually recover from norovirus within a few days, the disease is linked to about 900 deaths a year nationally — mostly involving people age 65 and older, according to the CDC.

Are there norovirus outbreaks in California?

There were 55 laboratory-confirmed norovirus outbreaks in California from January 2024 to November 2024, according to the most recent data obtained by the state Department of Public Health.

In comparison, California saw 73 outbreaks during the same 11-month period in 2023, the agency said, and 56 outbreaks during that period in 2022.

California saw the highest number of norovirus outbreaks in recent years in 2019, when a total of 76 outbreaks were reported from January and November.

Health care providers aren’t required to report individual cases of norovirus to local health departments, the California Department of Public Health said, but they are required to report outbreaks.

The California Public Health Department recently detected “elevated wastewater levels of norovirus” compared to December 2023 and January 2024, agency spokesman Grant Boyken told The Bee via email.

That’s based on data collected by WastewaterSCAN, a Stanford University-based research team that analyzes infectious diseases from wastewater treatment plants across the United States.

How is contagious virus treated? Is there a vaccine?

While there is no specific treatment to treat the illness, the CDC recommends drinking lots of water or other drinks that don’t have caffeine or alcohol, which dehydrate you.

You can also take over-the-counter medicines to help alleviate norovirus symptoms, including Imodium, Pepto-Bismol and Kaopectate, according to the National Institutes of Health.

As of Monday, Jan. 6, there was no vaccine against norovirus, Boyken said.

What’s the best way to avoid getting sick?

Since norovirus is highly contagious, the CDC recommends following these tips to avoid getting sick and spreading the illness:

•Wash your hands well and often.

•Cook shellfish thoroughly and wash fruits and vegetables.

•Clean and disinfect contaminated surfaces.

•Wash laundry in hot water.

•Stay home when sick for 48 hours after symptoms stop.

“Do not prepare or handle food for others when you are sick and for two days after you feel better,” Boyken said.

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