Contagious Stomach Bug on the Rise as Flu Season Fades

While flu season seems to be winding down, a particularly nasty and highly contagious stomach bug is making the rounds.

Recently, hikers in Southern California have reported norovirus cases, as noted by the Pacific Crest Trail Association. But it’s not just the outdoor enthusiasts who are feeling the effects.

Wastewater surveillance has shown a notable increase in norovirus levels across the country, particularly in the Northeast. From August 1 to May 7, the CDC’s NoroSTAT program recorded 1,194 outbreaks, which is a decrease from 2,534 during the same timeframe the previous year—aligning with past averages.

“Nationwide, norovirus remains in the HIGH category due to significant concentrations observed over the last three weeks,” reported Amanda Bidwell, the scientific program manager at WastewaterSCAN, a collaboration between Stanford University and Emory University.

Norovirus, often dubbed “winter vomiting disease,” can actually continue to cause outbreaks well into late spring, according to Bidwell.

Interestingly, wastewater data indicates a norovirus outbreak in the San Francisco Bay Area, though nationwide levels appear typical for this period, remarked Dr. Linda Yancey, an infectious disease expert at Memorial Hermann in Houston. “It’s just bad luck for California this time around,” she added.

Nationally, levels aren’t “unusually high” compared to previous seasons, as per the CDC. However, their tracking relies on confirmed cases from state agencies, which may not capture all illnesses, particularly since many individuals recover at home without seeking medical help.

“Monitoring norovirus in wastewater is crucial for understanding this very contagious virus, as clinical data on outbreaks is limited,” Bidwell mentioned.

Mutated Strain Evades Immunity

Currently, several norovirus strains are circulating, including GII.4—a strain that has been common for years—and GII.17, a more mutated strain that partially evades existing immunity. During the 2024–25 season, GII.17 surpassed GII.4 as the predominant strain in the U.S., accounting for about 75% of outbreaks.

Despite both variants causing similar symptoms, Yancey noted that the newer strain isn’t inherently more contagious. “It’s easier for it to spread because fewer people have partial immunity, hence they get sick and transmit the virus,” she explained.

Travel and Heat: Potential Factors

Although norovirus is generally more common in winter, there can be spikes in late spring, according to Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau in New York. The virus spreads rapidly among individuals, with infected people potentially transmitting it to as many as seven others, Yancey stated.

Seasonal travel and rising temperatures could contribute to these outbreaks. Increased holiday gatherings bring people into close quarters, promoting transmission, while hot weather tends to drive populations indoors, facilitating outbreaks in crowded spaces, noted Dr. Scott Roberts, associate medical director of infection prevention at Yale School of Medicine. “While I haven’t seen evidence of a new strain, evolution is a natural process for all viruses, and unfortunately, our immunity to norovirus doesn’t last long,” Roberts cautioned.

Understanding Norovirus Transmission

“Norovirus can be present all year, but in the Northern Hemisphere, it typically peaks from November through May,” Bidwell explained. It often spreads in crowded settings, like cruise ships, either through direct contact with infected individuals or by touching contaminated surfaces before touching one’s mouth.

People might contract the virus from contaminated food, especially when sick individuals prepare meals or drink untreated water. Those consuming ready-to-eat items, such as salads or sandwiches, face a higher risk.

Symptoms and Recommendations

Norovirus is the leading cause of foodborne illness in the U.S. Symptoms, typically manifesting within 12 to 48 hours of exposure, include vomiting, diarrhea, and abdominal cramps. Some might experience only one of these symptoms, but norovirus is particularly characterized by severe vomiting compared to other gastrointestinal issues.

Testing stool samples can confirm the virus, but doctors often diagnose it based on symptoms alone. Most healthy adults recover within a few days but can still spread the virus for up to two weeks after they feel better.

There’s no specific treatment for norovirus, and antibiotics are ineffective since it’s a viral infection, not bacterial. Experts recommend oral rehydration solutions instead of sports drinks to replenish lost nutrients, although mild dehydration may be addressed with beverages like Gatorade. In severe cases of dehydration, IV fluids may be necessary. Vulnerable groups, like young children and the elderly, should look for warning signs of dehydration, which include:

• Decreased urination
• Dry mouth
• Feeling lightheaded when standing
• Crying with few or no tears

To prevent norovirus, it’s advised to avoid raw shellfish (especially oysters) and to wash hands thoroughly with soap and water. Hand sanitizers often fall short against this virus, so cleaning surfaces with EPA-registered disinfectants specifically designed to kill norovirus is crucial.