Flu season may be mostly behind us, but a nasty, highly contagious stomach bug is rising.

In recent weeks, hikers in Southern California have been hit with norovirus, according to the Pacific Crest Trail Association.

Outdoor enthusiasts along the iconic trail aren’t the only ones suffering.

Recent wastewater surveillance reveals an upward trend to “high” levels of norovirus across much of the country, with rates currently rising in the Northeast.

From Aug. 1 to May 7, the Centers for Disease Control and Prevention’s NoroSTAT program recorded 1,194 outbreaks, compared with 2,534 during the same period the previous year — consistent with past averages.

“At the national level, norovirus is still in the HIGH category due to high concentrations over the last 21 days,” Amanda Bidwell, the scientific program manager at WastewaterSCAN, an academic program through Stanford University in partnership with Emory University, said in an email.

Norovirus is often called the “winter vomiting disease,” but it’s not unusual for outbreaks to continue through late spring, Bidwell said.

The wastewater numbers also highlight an outbreak of norovirus in the San Francisco Bay Area, although the numbers nationwide are average for this time of year, said Dr. Linda Yancey, an infectious disease specialist at Memorial Hermann in the Houston area.

“There really isn’t anything unusual about this one in California,” Yancey added. “They just got unlucky.”

National levels aren’t “unusually high” compared with prior seasons, according to the CDC, although the agency’s tracking is based on confirmed cases from state agencies and may not be picking up many illnesses.

“Monitoring norovirus in wastewater is very helpful for this highly contagious virus, as there is not a lot of clinical data to describe outbreaks because most people recover at home without seeing a healthcare professional,” Bidwell said.

Mutated strain evades immunity

Currently, several norovirus strains are circulating including GII.4, a common strain that has circulated for years, and GII.17, a more mutated strain that partially evades prior immunity because people haven’t been as exposed to it, Yancey said.

During the 2024–25 season, GII.17 surpassed GII.4 as the predominant strain in the U.S., causing about 75% of outbreaks.

Both variants cause very similar symptoms, even though the virus is known to mutate rapidly, Yancey said.

“The newer variant isn’t more contagious in and of itself,” she added. “It can spread more easily because fewer people have partial immunity to it, so they will get sick and spread the virus.”

How travel and the heat may be to blame

Although primarily a winter illness, norovirus can spike in the late spring, said Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau on Long Island, New York.

And the virus spreads easily between people, he added.

Like measles, norovirus is one of the most contagious known illnesses, with infected patients spreading the virus to up to seven other people on average, Yancey said.

Seasonal travel and extreme heat may also be to blame.

Increased holiday traffic brings large groups into close contact, accelerating viral transmission, while rising temperatures force people indoors, facilitating rapid spread in crowded rooms, said Dr. Scott Roberts, the associate medical director of infection prevention at Yale School of Medicine in Connecticut.

“I have not seen any evidence of a new strain, but as with all viruses, evolution occurs, and unfortunately, our immunity to norovirus is not long-lasting,” Roberts added.

How does norovirus spread?

“Norovirus can occur year-round, but in the Northern Hemisphere it is most commonly seen from November through May,” Bidwell said.

It commonly spreads in settings where many people are clustered together, like cruise ships, through direct contact with a sick person or by touching a contaminated surface and transferring it to the mouth.

People often get the infection from contaminated food, like when a sick cook prepares food and then serves it to people, or by drinking water that is not adequately treated.

Those who consume contaminated ready-to-eat cold foods, such as salads and sandwiches, are at higher risk.

Symptoms of norovirus and treatments to avoid

Norovirus is the most common cause of food poisoning in the U.S.

Vomiting, diarrhea and stomach cramps usually start 12 to 48 hours after exposure. Some people only complain of diarrhea or vomiting, but a hallmark of norovirus is intense vomiting compared to other stomach bugs.

It’s possible to test a person’s stool for the virus, but generally doctors diagnose the condition without testing.

Most healthy adults recover on their own within several days but still can spread the infection for up to two weeks, even after they feel better.

There is no specific treatment, and antibiotics don’t work since the infection is caused by a virus, not a bacterium.

Experts recommend oral rehydration fluids rather than sports drinks to replace important nutrients and minerals lost to vomiting and diarrhea — although drinks like Gatorade may help with mild dehydration.

But when some get too dehydrated, they may require intravenous fluids to replace what they’ve lost. Vulnerable populations, like young children and older adults, should monitor for warning signs of dehydration, including:

• Decreased urination
• Dry mouth
• Feeling dizzy when standing up
• Crying with minimal tears

To protect against norovirus, avoid raw shellfish (especially oysters) and always wash your hands thoroughly with soap and water. Hand sanitizers are often ineffective against this virus, so clean surfaces using EPA-registered disinfectants specifically labeled to kill it.