Is a 'vomiting virus' going around? Data shows spikes across US

Hikers along Southern California's Pacific Crest Trail have been smacked with fast-spreading cases of a "winter vomiting disease" – and they aren't the only ones.

At least two dozen hikers have fallen ill along the trail near Wrightwood in the San Gabriel Mountains of San Bernardino County over the last two weeks, according to the Victorville Daily Press, part of the USA TODAY Network, resulting in at least one air rescue. All are believed to have norovirus, a gastrointestinal illness that causes unpleasant symptoms.

While the hikers may be in one of the less ideal situations to contend with an illness known for causing vomiting and diarrhea, California is not the only home to a current outbreak.

Data from WastewaterSCAN, a Stanford University-led tool that tracks disease presence by testing wastewater samples nationwide, categorized norovirus activity as "high" and on an upward trend nationwide in late May. Norovirus was in 100% of samples taken in May, and concentrations were especially high in the Northeast and Midwest.

The latest CDC data from the National Respiratory and Enteric Virus Surveillance System also showed a recent, multi-week increase in cases after a sharp seasonal decline beginning in March. About 11.2% of administered norovirus tests came back positive the week ending April 28, while 12% did so the week ending May 2.

While the virus is typically associated with the cold winter months when flus and colds run rampant, these late-spring surges aren't abnormal.

Vomiting virus doesn't only surge during winter months

Between Aug. 1 and May 7, the CDC's NoroSTAT program tracked 1,194 norovirus outbreaks, which is within the normal range and consistent with the 2,534 reported during the same time in 2024-2025. The number of outbreaks falls about in the middle of the spectrum compared to the 2021-2025 seasons, said the CDC.

"These levels are generally on par with what we would expect this time of year," Amanda Bidwell, scientific program manager for WastewaterSCAN, told USA TODAY in an emailed statement. "It is typical to see norovirus concentrations start to increase late fall, reaching a peak in early March before decreasing in the spring."

The actual concentration of cases could be higher, however, as NoroSTAT only tracks confirmed cases reported by participating states. This is why wastewater can give us a better picture of the prevalence of illnesses not always easily detected in a healthcare setting, said Bidwell, especially because most people recover at home without seeing a doctor or being formally diagnosed.

"While norovirus continues to circulate as expected for this time of the year, rotavirus is the pathogen we are currently watching in wastewater because it can cause severe illness in children and is a vaccine-preventable disease," Dr. Alexandria Boehm, the Richard and Rhoda Goldman professor in environmental studies at Stanford University and WastewaterSCAN principal investigator, told USA TODAY.

Circulating strains also play a role in the spread. Two variations, GII.4 and GII.17, are currently the most common, with GII.4 having once been the dominant strain. The newer, more mutated GII.17 overtook GII.4 in the 2024-2025 season, accounting for 75% of cases, according to the CDC, and potentially, recent outbreaks driven by a lack of widespread immunity to the mutated strain.

What is norovirus? Why it's called the puking virus

Norovirus is a highly contagious virus that causes unpleasant gastrointestinal symptoms, often referred to as the "stomach bug" or "stomach flu," though it is not actually related to flu.

It is typically contracted from contaminated food or water, and is the most common cause of foodborne illness in the U.S., according to the Cleveland Clinic and the CDC. It is known for causing sudden, repeated vomiting and diarrhea.

Norovirus causes acute gastroenteritis, or inflammation of the stomach and intestines. Once someone contracts norovirus, their body sheds billions of tiny, contagious virus particles. Another person can become ill after coming in contact with even a few of these particles.

Usually, you'll stop being contagious about 48 hours after symptoms subside. You can continue shedding the virus in your stool for several weeks after recovery, according to the Mayo Clinic and Cleveland Clinic.

There are different strains of the virus; if your body builds up a response to one type, you can easily become ill again with another. Cases are most common during colder months, especially from November to April.

The only silver lining is that it is typically short-lived, lasting one to three days in the system.

Symptoms of norovirus, better known as the stomach bug

These are symptoms of norovirus infection, according to the Mayo Clinic and the Cleveland Clinic:

• Nausea and vomiting
• Diarrhea
• Stomach cramping or pain
• Sometimes, fever, headache or body aches may occur

Dehydration is a significant concern once you have contracted the virus. Symptoms indicating that your body may need more fluids include:

• Urinating less or having dark-colored urine
• Dry mouth
• Weakness
• Dizziness
• Headache

A person who is sick and has been infected with the virus will typically experience the symptoms suddenly. Children with the virus may be fussy and sleepy throughout the day, and cry without tears.

Symptoms usually appear 12 to 48 hours after exposure to the virus and last one to three days.

Though a person infected with the virus may experience unpleasant symptoms, others may be asymptomatic. However, anyone who's infected can still be contagious and pass the virus to others. Children younger than 5, older adults and people with weakened immune systems are more likely to develop severe infections, according to the CDC.

How does norovirus spread? How do you stop it?

Norovirus is easily spread via close contact with someone who is infected (even if they don't have symptoms), touching contaminated surfaces and then touching your mouth or nose, or, most commonly, eating or drinking contaminated food or beverages.

The virus can, and often does, spread when someone who is infected or has come into contact with contaminated surfaces touches food that is being prepared or served. The virus is difficult to kill, as it can withstand both hot and cold temperatures and many disinfectants.

One very simple but effective tip to avoid a weekend in the bathroom: Keep your hands clean. Wash your hands with hot, soapy water for at least 20 seconds and do it often. Wash them before and after eating, before and after handling food and after using the bathroom.

General food safety practices can also prevent catastrophe. Cook your food thoroughly and to the designated internal temperature, clean and sanitize frequently touched surfaces and objects, especially if they have been in contact with food, and wash fruits and vegetables before serving or eating them.