Norovirus infections are increasing earlier than expected this season across the United States, with particularly elevated activity being detected in parts of the Bay Area, according to recent wastewater monitoring data. Public health experts say the early rise is being driven in part by a new variant of the virus, leaving many people without prior exposure or immunity.
Data collected from wastewater surveillance systems show especially high levels of norovirus in the South Bay, Redwood City, and San Francisco. Wastewater monitoring has become an increasingly important tool for tracking community health trends, as it can identify viral activity before large numbers of people seek medical care. In this case, the data suggest that norovirus is circulating widely weeks earlier than is typically seen during the winter illness season.
Norovirus, often referred to as the stomach flu, is a highly contagious virus that causes gastrointestinal illness. Common symptoms include vomiting, stomach cramps, nausea, and diarrhea, which can begin suddenly and be severe. While the illness is usually short-lived, it can be particularly dangerous for young children, older adults, and people with weakened immune systems due to the risk of dehydration.
Health officials note that the emergence of a new norovirus variant may be contributing to the spike in cases. Because many individuals have not previously encountered this strain, population-level immunity is likely lower, allowing the virus to spread more easily. Unlike influenza, there is no vaccine for norovirus, making prevention measures especially important during periods of increased transmission.
The virus spreads through close contact with an infected person, consuming contaminated food or water, or touching contaminated surfaces and then touching the mouth. Norovirus is known for its ability to persist on surfaces and for requiring only a small number of viral particles to cause infection. People can also continue to shed the virus for several days after symptoms have resolved, increasing the likelihood of unknowingly spreading it to others.
Public health authorities are urging residents to take precautions as cases rise. Frequent handwashing with soap and water remains one of the most effective ways to reduce transmission, as alcohol-based hand sanitizers are less effective against norovirus. Proper food handling, thorough cleaning of surfaces, and staying home while sick are also critical steps to limit the spread.
The early increase in norovirus activity has prompted concern among health professionals, who warn that outbreaks could intensify as people spend more time indoors during cooler months. Wastewater data will continue to be closely monitored to assess trends and provide early warnings of further increases.
While most people recover from norovirus without medical treatment, officials emphasize the importance of recognizing symptoms early and taking steps to prevent dehydration. As the season progresses, health experts say awareness and basic hygiene practices will be key to minimizing the impact of this highly contagious virus on communities across the Bay Area and beyond.
