Autumn brings the arrival of cold and flu season to the Bay Area – and with it, higher rates of a contagious stomach bug that’s infamous for causing vomiting and diarrhea.
Norovirus, often called the stomach flu, spreads in North America from November to April. The virus is less deadly than the flu or COVID-19, but it can cause infants and older adults to become dangerously dehydrated. Most people soldier through several days of terrible stomach issues and recover on their own.
Unfortunately, some public health experts said the virus may spread faster this season. That’s because a new strain of the virus appeared last year, spurring more outbreaks. This new strain is more contagious, according to the Centers for Disease Control and Prevention. It is probably driving the high transmission rates in some parts of the Bay Area, said Peter Chin-Hong, a UCSF professor of microbiology and immunology.
Here’s what the stomach bug means for the holidays and how you can reduce your chance of catching it:
Q: What is the new strain, and how is it different?
Until last year, norovirus hadn’t mutated in a major way for about 30 years, Chin-Hong said. That meant that our immune systems were more familiar with the virus and could fight it off more effectively.
But last year, a new variety, GII.17, became the most common strain of norovirus in the U.S. That strain is more contagious and drove a spike in norovirus outbreaks nationally last winter, according to CDC data. The agency’s researchers think it originated from a 2021 outbreak in Romania.
From autumn 2023 to spring 2024, Santa Clara County officials confirmed two norovirus outbreaks. The following virus season, as the new variant spread in 2024 and 2025, brought 16 outbreaks, according to a county public health spokesperson.
Q: Is the virus spreading in the Bay Area?
Wastewater testing shows high concentrations of the virus in Silicon Valley, San Francisco and Vallejo. But public health officials aren’t sounding the alarm.
Chin-Hong said evidence of the virus typically appears first in wastewater. Then, people start getting sick and occasionally go to the hospital.
Santa Clara County has seen no norovirus outbreaks reported to the public health department in the past three months, according to a spokesperson, and wastewater data shows that the virus is less prevalent than a peak last winter.
In Contra Costa County, transmission “appears consistent with what we typically see this time of year, based on a low number of reported outbreaks and current lab surveillance data,” a spokesperson said in an email.
The county’s public health experts can’t predict how the new strain will affect residents, but “it has been associated with more severe illness,” the spokesperson said.
Chin-Hong expects to see the virus spread more this month as holiday celebrations continue.
Q: How does the virus spread, and how can you stay healthy?
Unlike Covid-19 and the flu, which experts say generally require exposure to hundreds or thousands of viral particles to cause infection, norovirus can make someone sick with as few as five to 10 particles. Transmission can spike around the holidays, Stanford Professor of Medicine and Pediatrics Dr. Dean Winslow said, as people gather in homes.
“These outbreaks of winter vomiting illnesses are nothing new,” Winslow said.
Norovirus spreads when microscopic particles from an infected person’s feces or vomit enter the mouth, often through contaminated hands, surfaces, food or water. Hand washing is one of the most recommended preventative measures; hand sanitizer does not work well against the virus.The CDC also recommends handling and preparing food safely by washing produce well, thoroughly sanitizing cookware, and routinely disinfecting home surfaces and washing clothes in hot water if you suspect they have norovirus particles on them.
Q: You’ve got norovirus. What now?
If someone is ill, Winslow said it is important to stay home, wash hands, and clean surfaces, especially in bathrooms where vomiting might occur. He also recommends drinking water with electrolytes to replace the minerals lost through vomiting and diarrhea.“Staying hydrated is the most important thing,” he said.
Fortunately, Winslow says the duration of illness is relatively short, with symptoms generally lasting around one to three days and peaking in the beginning.
