SACRAMENTO, Calif. — Rising levels of noroviruses, a highly contagious infection, have been detected in wastewater in several California cities, testing shows.

WastewaterSCAN data shows “high concentrations” of noroviruses in Sacramento, Davis, San Francisco, Sunnyvale, Palo Alto, Vallejo, Novato, Redwood City, Turlock and San Rafael.

Moderate concentrations were found in San Jose, Napa and Merced.

Noroviruses, also known as the “stomach flu,” are a highly contagious cause of acute stomach and intestinal infections, producing diarrhea, cramps and vomiting that can lead to dehydration.

Here’s what to know:

What are noroviruses?

Noroviruses are the most common cause of acute stomach and intestinal infections in the United States, reported the U.S. Food and Drug Administration.

Infections also are sometimes called stomach flu, viral gastroenteritis or the winter vomiting bug.

The U.S. reports 19 million to 21 million cases a year. Humans are the only hosts of the virus.

The virus was formerly known as the Norwalk virus, because the first known outbreak took place at an elementary school in Norwalk, Ohio, according to norovirus.com.

Scientists identified the virus in 1972 from stool samples stored after the outbreak. It was officially renamed norovirus by the International Committee of Taxonomy of Viruses in 2002.

How does a norovirus spread?

Extremely. The U.S. Centers for Disease Control and Prevention caution that noroviruses can be transmitted by infected people, contaminated food or water, or just by touching contaminated surfaces.

People with a norovirus are most contagious during the illness and for a few days afterward, and the virus can remain in stool for up to two weeks after the illness. The virus can survive temperature extremes, too.

Also, catching a norovirus doesn’t help you fight it off later, in part because there are many different types of noroviruses — catching one doesn’t protect you from the others.

​Where do noroviruses spread?

Cruise ships, nursing homes, daycare centers and schools are common breeding grounds for norovirus — basically, anywhere large numbers of people are packed in close quarters, the CDC reported.

Outbreaks on cruise ships frequently make the news and there are countless travel websites dedicated to tracking cruise lines with the worst records for the illness.

What are the symptoms of a norovirus?

Diarrhea, cramps and vomiting usually start within 12 to 48 hours of exposure to the virus, said the Mayo Clinic.

Norovirus symptoms normally last one to three days, and most people recover without treatment. But infants, older adults and people with chronic illnesses may require medical attention for dehydration.

The Mayo Clinic advised that people with norovirus take special care to replace fluids lost by vomiting or diarrhea to prevent dehydration.

Drinks like Pedialyte are good for young children, while sports drinks and broths are suggested for adults. But sugary drinks, like sodas and fruit juices, can make diarrhea worse, while alcohol or caffeinated drinks can speed dehydration.

Soup, bananas, yogurt and broiled vegetables are good choices to help reduce vomiting.

What are the complications of a norovirus?

Possible norovirus complications include dehydration and renal failure, the National Institutes of Health said.

Other possible complications involve hyponatremia, in which sodium levels in the blood drop too low, metabolic alkalosis, in which acid levels become unbalanced, and hemolytic uremic syndrome, in which red blood cells and platelets are destroyed.

When should you see a doctor about a norovirus?

See a doctor if you or someone in your family has symptoms for more than three days, bloody stool or vomit, lack of urine or dark urine, which may mean dehydration, or an oral temperature of over 101.5 degrees.

Go to the emergency room if you or a family member has a temperature over 102 degrees for more than two days that doesn’t respond to medicine, severe dehydration or dizziness, or lightheadedness on standing.

How can you avoid noroviruses?

Good hygiene is the key to avoiding noroviruses, suggested WebMD.com. Wash your hands frequently with soap and water, particularly after using the bathroom and before preparing food. Alcohol-based cleaners are not as effective.

The site also advises carefully throwing away contaminated items, such as dirty diapers.

Wash raw fruits and vegetables, and cook oysters and other shellfish thoroughly.

Clean and disinfect surfaces with a mixture of detergent and chlorine bleach after someone’s sick, WebMD said. And if you catch a norovirus, don’t prepare food for at least two to three days after you feel better.

How do you treat a norovirus?

Since it’s a virus, antibiotics aren’t any help, and there are no antiviral drugs for noroviruses.

Resting and drinking extra liquids to help restore fluids lost to diarrhea and vomiting can help, UC Davis Health said.

When symptoms are severe, health professionals may provide intravenous fluids to treat dehydration.

____

©2026 The Sacramento Bee. Visit at sacbee.com. Distributed by Tribune Content Agency, LLC.