A highly contagious stomach bug is spreading through much of the United States, and wastewater surveillance data suggests the situation is more widespread than confirmed case counts alone would indicate. Norovirus levels have reached the high category nationally, according to WastewaterSCAN, an academic monitoring program run through Stanford University in partnership with Emory University.
The Northeast and Midwest are currently seeing the most elevated concentrations. The West and South remain in the medium category. While norovirus is commonly associated with winter, public health experts say it is typical for outbreaks to continue through late spring, and this year is following that pattern.
From August 1, 2025 to May 7, 2026, the Centers for Disease Control and Prevention’s NoroSTAT program recorded 1,194 outbreaks across participating states. That figure is lower than the 2,534 reported during the same period the previous year, though experts say it remains consistent with historical averages.
A mutated strain is making more people vulnerable
Two strains are currently circulating. The first is GII.4, a well-known variant that has been present for years. The second is GII.17, a more mutated strain that is proving harder for the immune system to recognize because fewer people have prior exposure to it.
During the 2024 to 2025 season, GII.17 overtook GII.4 as the dominant strain in the country, accounting for roughly 75% of outbreaks. Both strains produce similar symptoms, but GII.17 spreads more easily because immunity against it is limited across the general population. Infectious disease specialists note that the strain is not inherently more contagious. It simply finds more susceptible hosts.
Norovirus is among the most contagious illnesses known to medicine. On average, one infected person passes the virus to up to seven others.
Hikers in California were among the first to feel it this season
The current spread drew early attention after hikers along the Pacific Crest Trail in Southern California began reporting symptoms. The Pacific Crest Trail Association confirmed it was aware of gastrointestinal illness cases in the Wrightwood area and said it was working with San Bernardino County public health officials to assess the situation.
At least two dozen hikers reported symptoms along a section of trail near Los Angeles, and at least seven were diagnosed with norovirus. One man, a 73-year-old hiker, was airlifted to a hospital after becoming severely weakened by the illness. A separate outbreak was also identified in the San Francisco Bay Area, though infectious disease specialists described both situations as unfortunate rather than unusual.
How norovirus spreads and what to watch for
The virus moves quickly in crowded settings. Cruise ships, shared kitchens, and indoor gatherings are common transmission environments. It spreads through direct contact with an infected person, through contaminated surfaces, and through food or water that has not been properly handled. Ready-to-eat cold foods such as salads and sandwiches carry a higher risk when prepared by someone who is sick.
Symptoms typically appear between 12 and 48 hours after exposure and include vomiting, diarrhea, and stomach cramps. Most healthy adults recover within a few days, but infected individuals can continue spreading the virus for up to two weeks after symptoms resolve.
There is no specific treatment. Antibiotics are ineffective because the illness is viral. Oral rehydration fluids are the recommended approach to managing dehydration, and sports drinks may help in mild cases. Intravenous fluids are sometimes needed when dehydration becomes severe, particularly in young children and older adults.
Hand sanitizer offers limited protection against norovirus. Thorough handwashing with soap and water remains the most reliable defense, along with EPA-registered disinfectants on surfaces and avoiding raw shellfish, particularly oysters.
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