The dreaded norovirus — the “vomiting bug” that often causes stomach flu symptoms — is climbing again in California, and doctors warn that a new subvariant could make even more people sick this season.

In L.A. County, concentrations of norovirus are already on the rise in wastewater, indicating increased circulation of the disease, the local Department of Public Health told the Los Angeles Times.

Norovirus levels are increasing across California, and the rise is especially notable in the San Francisco Bay Area and L.A., according to the California Department of Public Health.

And the rate at which norovirus tests are confirming infection is rising nationally and in the Western U.S. For the week that ended Nov. 22, the test positivity rate nationally was 11.69%, up from 8.66% two months earlier. In the West, it was even worse: 14.08%, up from 9.59%, according to the U.S. Centers for Disease Control and Prevention.

Norovirus is extraordinarily contagious, and is America’s leading cause of vomiting and diarrhea, according to the CDC. Outbreaks typically happen in the cooler months between November and April.

Clouding the picture is the recent emergence of a new norovirus strain — GII.17. Such a development can result in 50% more norovirus illness than typical, the CDC says.

“If your immune system isn’t used to something that comes around, a lot of people get infected,” said Dr. Peter Chin-Hong, an infectious diseases expert at UC San Francisco.

During the 2024-25 winter season, GII.17 overthrew the previous dominant norovirus strain, GII.4, that had been responsible for more than half of national norovirus outbreaks over the preceding decade. The ancestor of the GII.17 strain probably came from a subvariant that triggered an outbreak in Romania in 2021, according to CDC scientists.

GII.17 vaulted in prominence during last winter’s norovirus surge and was ultimately responsible for about 75% of outbreaks of the disease nationally.

The strain’s emergence coincided with a particularly bad year for norovirus, one that started unusually early in October 2024, peaked earlier than normal the following January and stretched into the summer, according to CDC scientists writing in the journal Emerging Infectious Diseases.

During the three prior seasons, when GII.4 was dominant, norovirus activity had been relatively stable, Chin-Hong said.

Norovirus can cause substantial disruptions — as many parents know all too well. An elementary school in Massachusetts was forced to cancel all classes on Thursday and Friday because of the “high volume of stomach illness cases,” which was suspected to be driven by norovirus.

More than 130 students at Roberts Elementary School in Medford, Mass., were absent Wednesday, and administrators said there probably wouldn’t be a “reasonable number of students and staff” to resume classes Friday. A company was hired to perform a deep clean of the school’s classrooms, doorknobs and kitchen equipment.

Some places in California, however, aren’t seeing major norovirus activity so far this season. Statewide, while norovirus levels in wastewater are increasing, they still remain low, the California Department of Public Health said.

There have been 32 lab-confirmed norovirus outbreaks reported to the California Department of Public Health so far this year. Last year, there were 69.

Officials caution the numbers don’t necessarily reflect how bad norovirus is in a particular year, as many outbreaks are not lab-confirmed, and an outbreak can affect either a small or large number of people.

Between Aug. 1 and Nov. 13, there were 153 norovirus outbreaks publicly reported nationally, according to the CDC. During the same period last year, there were 235.

UCLA hasn’t reported an increase in the number of norovirus tests ordered, nor has it seen a significant increase in test positivity rates. Chin-Hong said he likewise hasn’t seen a big increase at UC San Francisco.

“Things are relatively still stable clinically in California, but I think it’s just some amount of time before it comes here,” Chin-Hong said.

In a typical year, norovirus causes 2.27 million outpatient clinic visits, mostly young children; 465,000 emergency department visits, 109,000 hospitalizations, and 900 deaths, mostly among seniors age 65 and older.

People with severe ongoing vomiting, profound diarrhea and dehydration may need to seek medical attention to get hydration intravenously.

“Children who are dehydrated may cry with few or no tears and be unusually sleepy or fussy,” the CDC says. Sports drinks can help with mild dehydration, but what may be more helpful are oral rehydration fluids that can be bought over the counter.

Children under the age of 5 and adults 85 and older are most likely to need to visit an emergency room or clinic because of norovirus, and should not hesitate to seek care, experts say.

“Everyone’s at risk, but the people who you worry about, the ones that we see in the hospital, are the very young and very old,” Chin-Hong said.

Those at highest risk are babies, because it doesn’t take much to cause potentially serious problems. Newborns are at risk for necrotizing enterocolitis, a life-threatening inflammation of the intestine that virtually only affects new babies, according to the National Library of Medicine.

Whereas healthy people generally clear the virus in one to three days, immune-compromised individuals can continue to have diarrhea for a long time “because their body’s immune system can’t neutralize the virus as effectively,” Chin-Hong said.

The main way people get norovirus is by accidentally drinking water or eating food contaminated with fecal matter, or touching a contaminated surface and then placing their fingers in their mouths.

People usually develop symptoms 12 to 48 hours after they’re exposed to the virus.

Hand sanitizer does not work well against norovirus — meaning that proper handwashing is vital, experts say.

People should lather their hands with soap and scrub for at least 20 seconds, including the back of their hands, between their fingers and under their nails, before rinsing and drying, the CDC says.

One helpful way to keep track of time is to hum the “Happy Birthday” song from beginning to end twice, the CDC says. Chin-Hong says his favorite is the chorus of Kelly Clarkson’s “Since U Been Gone.”

If you’re living with someone with norovirus, “you really have to clean surfaces and stuff if they’re touching it,” Chin-Hong said. Contamination is shockingly easy. Even just breathing out little saliva droplets on food that is later consumed by someone else can spread infection.

Throw out food that might be contaminated with norovirus, the CDC says. Noroviruses are relatively resistant to heat and can survive temperatures as high as 145 degrees.

Norovirus is so contagious that even just 10 viral particles are enough to cause infection. By contrast, it takes ingesting thousands of salmonella particles to get sick from that bacterium.

People are most contagious when they are sick with norovirus — but they can still be infectious even after they feel better, the CDC says.

The CDC advises staying home for 48 hours after infection. Some studies have even shown that “you can still spread norovirus for two weeks or more after you feel better,” according to the CDC.

The CDC also recommends washing laundry in hot water.

Besides schools, other places where norovirus can spread quickly are cruise ships, day-care centers and prisons, Chin-Hong said.

The most recent norovirus outbreak on a cruise ship reported by the CDC is on the ship AIDAdiva, which set sail on Nov. 10 from Germany. Out of 2,007 passengers on board, 4.8% have reported being ill. The outbreak was first reported on Nov. 30 following stops that month at the Isle of Portland, England; Halifax, Canada; Boston; New York City; Charleston, S.C.; and Miami.

According to CruiseMapper, the ship was set to make stops in Puerto Vallarta on Saturday, San Diego on Tuesday, Los Angeles on Wednesday, Santa Barbara on Thursday and San Francisco between Dec. 19-21.