The dreaded norovirus – a “vomit virus” that often causes stomach flu-like symptoms – is spreading again in California, and doctors are warning that a new variant could sicken even more people this season.
In Los Angeles County, concentrations of norovirus in wastewater are already rising, indicating increased circulation of the disease, the local public health department told the Los Angeles Times.
According to the California Department of Public Health, norovirus levels are rising throughout California, and the rise is particularly noticeable in the San Francisco Bay Area and Los Angeles.
And the rate at which norovirus tests confirm infection is rising across the country and in the western United States. For the week ending November 22, the country's test positivity rate was 11.69%, down from 8.66% two months earlier. In the West, things were even worse: 14.08% versus 9.59%, according to US Centers for Disease Control and Prevention.
Norovirus is extremely contagious and is the leading cause of vomiting and diarrhea in America, according to the Centers for Disease Control and Prevention (CDC). Outbreaks typically occur during the cooler months of November through April.
The picture is clouded by the recent emergence of a new strain of norovirus – GII.17. According to the CDC, this development could result in a 50% increase in norovirus incidence compared to normal.
“If your immune system isn't used to something that's going on around you, a lot of people get infected,” said Dr. Peter Chin-Hong, an infectious disease expert at the University of California, San Francisco.
During the 2024–2025 winter season, GII.17 displaced the previous dominant norovirus strain, GII.4, which was responsible for more than half of the nation's norovirus outbreaks over the previous decade. The ancestor of the GII.17 strain likely evolved from a subvariant that sparked the outbreak in Romania in 2021, according to CDC scientists.
GII.17 rose to prominence during the norovirus surge last winter and ultimately accounted for about 75% of the country's outbreaks.
The emergence of the strain coincided with a particularly bad year for norovirus, which began unusually early in October 2024, peaked earlier than usual the following January and stretched into the summer, according to CDC scientists writing in the journal. New infectious diseases.
During the previous three seasons when GII.4 dominated, norovirus activity was relatively stable, Chin-Hong said.
Norovirus can cause serious disruption, as many parents know all too well. An elementary school in Massachusetts was forced to cancel all classes Thursday and Friday due to a “high number of stomach illnesses” believed to be caused by norovirus.
More than 130 students at Roberts Elementary School in Medford, Massachusetts, were absent Wednesday, and administrators said there likely won't be a “reasonable number of students and staff” to resume classes Friday. A company was hired to deep clean the school's classrooms, door handles and kitchen equipment.
However, some places in California have not yet seen significant norovirus activity this season. Although norovirus levels in wastewater are increasing, they remain low across the state, according to the California Department of Public Health.
There have been 32 laboratory-confirmed norovirus outbreaks reported to the California Department of Public Health this year. Last year there were 69 of them.
Officials caution that the numbers do not necessarily reflect how dangerous norovirus is in a given year, since many outbreaks are not laboratory confirmed and an outbreak can affect small or large numbers of people.
There were 153 publicly reported norovirus outbreaks nationally between Aug. 1 and Nov. 13, according to the CDC. During the same period last year there were 235.
UCLA did not report an increase in the number of norovirus tests ordered and did not see a significant increase in test positivity rates. Chin-Hong said he hasn't seen much growth at UCSF either.
“In California, the situation is relatively stable clinically, but I think it will be a while before it gets here,” Chin-Hong said.
In a typical year, norovirus causes 2.27 million outpatient visits, mostly in young children; There were 465,000 emergency department visits, 109,000 hospitalizations and 900 deaths, mostly among seniors aged 65 and older.
People with severe ongoing vomiting, profound diarrhea, and dehydration may need to seek medical attention for intravenous fluids.
“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 oral rehydration fluids, which can be purchased over the counter, may be more helpful.
Children under age 5 and adults 85 and older are most likely to need to go to the emergency room or clinic because of norovirus, and should not hesitate to seek help, experts say.
“Everyone is at risk, but the people you worry about, the ones we see in the hospital, are the very young and the very old,” Chin-Hong said.
Infants are most at risk because it doesn't take long for it to cause potentially serious problems. Newborns are at risk of necrotizing enterocolitis, a life-threatening intestinal inflammation that is almost exclusively influences newborns, according to the National Library of Medicine.
While healthy people usually clear the virus within one to three days, people with weakened immune systems may continue to have diarrhea for a long time “because their body's immune system cannot effectively neutralize the virus,” Chin-Hong said.
The main way you get norovirus is by accidentally drinking water or food contaminated with feces, or by touching a contaminated surface and then putting your fingers in your mouth.
Symptoms in people usually appear 12 to 48 hours after exposure to the virus.
Hand sanitizer is not effective against norovirus, meaning proper hand washing is vital, experts say.
People should lather their hands with soap and scrub them for at least 20 seconds, including the backs of their hands, between their fingers and under their nails, before rinsing and drying. CDC speaks.
According to the CDC, one helpful way to keep track of time is to sing the “Happy Birthday” song from start to finish twice. Chin-Hong says his favorite chorus is from the Kelly Clarkson song “Since you've been gone”
If you live with someone infected with norovirus, “you're really going to have to clean surfaces and things like that if they touch them,” Chin-Hong said. Pollution it's amazingly easy. Even simply breathing small droplets of saliva onto food that is later eaten by someone else can spread the infection.
The Centers for Disease Control and Prevention (CDC) advises throwing away food that may be contaminated with norovirus. Noroviruses are relatively heat resistant and can withstand temperatures up to 145 degrees.
Norovirus is so contagious that it only takes 10 viral particles to become infected. On the contrary, to become ill with this bacterium, you must ingest thousands of Salmonella particles.
People are most contagious when they are sick with norovirus, but they can still be contagious even after they feel better, the CDC says.
The Centers for Disease Control and Prevention advises stay home 48 hours after infection. Some studies have even shown that “you can still spread norovirus for two weeks or more after you feel better.” CDC.
The CDC also recommends washing clothes in hot water.
Besides schools, other places where norovirus can spread quickly include cruise ships, day care centers and prisons, Chin-Hong said.
Most recent norovirus outbreak reported on a cruise ship CDC is on the ship AIDAdivasailed November 10th from Germany. Of the 2,007 passengers on board, 4.8% reported illness. The outbreak was first reported on November 30, following a shutdown that month on the Isle of Portland in England; Halifax, Canada; Boston; New York; Charleston, South Carolina; and Miami.
According to CruiseMapperThe ship was scheduled to make stops in Puerto Vallarta on Saturday, San Diego on Tuesday, Los Angeles on Wednesday, Santa Barbara on Thursday and San Francisco from December 19 to 21.
