Norovirus Cases Spike in SF, Marin & Silicon Valley

Wastewater surveillance data is flashing warning signs across the Bay Area this week, with norovirus levels spiking in communities stretching from western San Francisco to Silicon Valley.

Data from WastewaterSCAN, a public health monitoring program that tracks pathogens in sewage systems, shows high concentrations of norovirus detected in wastewater serving the western side of San Francisco, Central Marin County, Novato, and Redwood City. Moderate levels have turned up in east SF, Santa Rosa, Vallejo, San Jose, and Palo Alto. The geographic spread suggests this is not a localized cluster but a regional surge hitting communities on both sides of the bay.

Norovirus, the highly contagious gastrointestinal virus commonly called the “stomach bug,” causes vomiting, diarrhea, and stomach cramps, often accompanied by fever, headache, and body aches. Symptoms typically appear 12 to 48 hours after exposure, according to the CDC. The illness is unpleasant for healthy adults but genuinely dangerous for small children and elderly people, who are most vulnerable to the dehydration it triggers. The CDC recommends drinking plenty of fluids, including electrolyte beverages and oral rehydration drinks for more severe cases.

Dr. Monica Gandhi, an infectious disease specialist at UC San Francisco, put the transmission risk bluntly. “It is extremely contagious,” she said. “And people don’t wash their hands, especially kids.” The virus spreads through contaminated surfaces and food, and through direct contact with infected individuals. The CDC also flags raw shellfish, particularly oysters, as a known vector. Given that the Bay Area has no shortage of oyster enthusiasts, that warning is worth keeping in mind.

The regional spike arrives amid a broader national uptick in norovirus activity. The virus has been tearing through cruise ships at a notable rate. Last week, more than 150 people aboard a Princess cruise ship contracted the illness out of roughly 7,000 passengers and crew, on a voyage that originated in Fort Lauderdale and sailed through the Caribbean. In February, 27 people became ill aboard the luxury Regent Seven Seas ship during a Miami-to-Honolulu crossing. In late 2025, more than 80 passengers and crew contracted norovirus on a Holland America ship, also departing Fort Lauderdale.

The cruise ship pattern reflects a broader trend that has been building. According to Global Biodefense, 2024 saw 16 cruise ships report outbreaks of gastrointestinal disease, including five in December of that year alone, reportedly the largest single-month total in over a decade. The enclosed, high-traffic environment of a cruise ship is essentially an ideal incubation chamber for norovirus, where contaminated surfaces, shared dining spaces, and close quarters accelerate transmission.

The same dynamics apply to any densely populated setting: offices, schools, public transit. San Francisco’s Caltrain commuters, BART riders, and Mission District residents sharing taqueria counter space all face the same basic risk. Norovirus can survive on hard surfaces for days and is resistant to many common disinfectants, which is part of why it spreads so efficiently even in settings with basic sanitation.

The WastewaterSCAN data provides an early signal that health officials can act on before emergency rooms fill up. That kind of surveillance infrastructure has real value, giving public health departments a heads-up rather than a hindsight report. Whether local agencies use this data to ramp up cleaning in transit stations or issue targeted advisories to food service workers is the more important question.

For now, the practical advice is straightforward: wash hands thoroughly with soap and water, especially before handling food and after using the bathroom. Alcohol-based hand sanitizers are less effective against norovirus than soap and water. Sick individuals should stay home from work and keep children out of school. Anyone who ate raw oysters recently and feels off should monitor symptoms closely.

The wastewater data will keep updating. Spring gatherings, farmers markets, and outdoor dining season are picking back up this month. This is a poor time to get sloppy about hand hygiene.