FILE: An aerial view of San Francisco.

In addition to grappling with a brutal heat wave, Northern Californians are now facing another common enemy: norovirus. The virus is also known colloquially as the stomach flu, but the illness, which is often foodborne, isn’t caused by the influenza virus, and the symptoms manifest much differently.  

According to WastewaterSCAN, traces of the contagious gastrointestinal bug are high in western San Francisco, Marin, Novato, Palo Alto and Sunnyvale, and moderate in Redwood City, San Jose and eastern portions of San Francisco. The health organization monitors traces of several different viruses, including COVID-19, in wastewater treatment facilities throughout the U.S. to give the public a broader idea of when they’re actually circulating.  

Article continues below this ad

Norovirus causes vomiting, diarrhea and painful stomach cramps, and can infect people of all ages. Other flu-like symptoms include fever, headache and body aches, and can lead to dehydration, especially among small children and elderly adults. In most cases, people get sick when coming into contact with the virus while caring for someone who’s contagious, sharing food with them or touching contaminated surfaces. And though the balmy weather may call for it, raw oysters can also contain the virus, the Centers for Disease Control and Prevention warns. 

“It is extremely contagious,” Dr. Monica Gandhi, an infectious disease specialist at UC San Francisco, told SFGATE on Monday morning, adding that hand-washing is crucial to prevent the spread. “And people don’t wash their hands, especially kids.”

Previously thought to be associated with cruise ships and preschools, once the virus is circulating, it can quickly become an outbreak, she continued. Norovirus outbreaks often begin “at salad bars and cruise ships and places where people congregate closely,” Gandhi said, meaning they can occur in hot or cold weather — so if you’re sharing charcuterie at an outdoor picnic or cramming inside an air-conditioned movie theater to escape the heat, you’re equally at risk of contracting the virus. It’s also possible to spread it from 48 hours to up to two weeks or more after you feel better. 

Norovirus outbreaks are such a problem on confined spaces like cruise ships, for instance, that the CDC publicly tracks them. So far this year, over 100 people fell ill on a Princess Cruise in March, and in 2024, nearly 160 got sick on a Cunard cruise headed from San Francisco to Hawaii, leading to the isolation of sick passengers and employees. 

Article continues below this ad

Don't let Google decide who you trust.

To avoid contracting the illness, the CDC recommends frequently washing your hands, cooking shellfish thoroughly and staying home when sick. More information about preventing the spread of norovirus can be found on the agency’s website. 

— Notorious cruise ship stomach bug rages in Northern California— Arrest made in killing of Calif. woman who livestreamed sister's death— 2 California ski resorts close early for the season— Calif. man dies after falling on rattlesnake while mountain biking

Sign up for daily SFGATE breaking news alerts here.

© SFGate