As this winter's tripledemic respiratory virus surge winds down and gives way to allergy season, some doctors are cautioning about another highly contagious virus spreading across the United States and sickening children: norovirus.

Cases and outbreaks of the notorious wintertime stomach bug have been steadily rising in the U.S. since last fall, more recently spiking and reaching new highs for this season, according to the most recent surveillance data from the U.S. Centers for Disease Control and Prevention.

Norovirus — commonly known as the “stomach flu” — is actually a group of viruses that cause acute gastroenteritis, an inflammation of the lining of the stomach and intestines, which leads to acute vomiting and diarrhea, per the CDC. Unlike its nickname suggests, norovirus is not related to the flu or influenza viruses.

In the U.S., the percentage of norovirus tests coming back positive, averaged over three weeks, was over 18% for the week ending on March 25, which surpasses last year's peak, according to the most recent data from the CDC's National Respiratory and Enteric Virus Surveillance System (NREVSS).

Outbreaks of norovirus are also on the rise in the 14 states reporting data to CDC’s NoroSTAT program, Kate Grusich, CDC spokesperson, tells TODAY.com in a statement.

"The most recent CDC data .... show that reported cases from state health departments and clinical laboratories are increasing, but still remain within the expected range for this time of year," Grusich says, adding that the U.S. typically sees about 2,500 outbreaks per year.

However, some experts tell TODAY.com they're still bracing for norovirus outbreaks, especially among school-age children.

In late March, hundreds of children at a high school in Long Island, New York, were out sick with what officials believe was norovirus linked to an outbreak at a school dance, NBC New York reported.

Charlottesville, Virginia, is also facing a norovirus outbreak, local outlet NBC 28 reported.

The situation across the pond doesn't ease concerns either. Earlier this month, health officials in England warned that norovirus cases had reached the highest levels for this time of year in over a decade. In mid-March, reports of norovirus were 77% higher than the five-season average during the same period, according to the U.K. Health Security Agency.

So what can we expect for spring in the U.S.? We spoke to experts about how this year's norovirus season may compare to others and what you need to know about symptoms, transmission, treatment and prevention.

Why is norovirus surging this year?

Norovirus outbreaks are common in the U.S., says Grusich. Each year, norovirus causes 19 to 21 million cases of vomiting and diarrhea, 109,000 hospitalizations and 900 deaths, per the CDC.

Although norovirus can spread year-round, it has a wintertime seasonality in the U.S., says Grusich, so cases tend to peak during the colder months. The vast majority of outbreaks occur between November and April, according to the CDC.

While it is too early to tell exactly when exactly norovirus will peak in the U.S. and how the 2022-2023 will compare to prepandemic seasons, it is clear that cases and outbreaks are rising right now.

“It’s going up quickly right now. It’s not yet at the peak we’ve seen in previous years, but it’s definitely on the rise and pretty suddenly in the past few weeks," Dr. Ali Alhassani, a pediatrician at Boston Children’s Hospital and head of clinical at Summer Health, tells TODAY.com.

A surge in norovirus outbreaks could lead to an increased number of emergency room visits, Alhassani notes, or a reduction in the already-strained health workforce.

Dr. Luis Ostrosky, an infectious disease specialist at UTHealth Houston and Memorial Hermann in Houston, tells TODAY.com that “norovirus activity is higher and earlier than usual (right now), but definitely not a big outlier compared to pre-pandemic levels.”

Children who managed to evade the virus over the past few years are now being exposed to norovirus and many other viruses that tend to peak and circulate among schools at this time of year.

"We've always had seasonal increases and waves of norovirus, and our hospitals would be filled with kids," Dr. Albert Ko, infectious disease physician and professor of public health, epidemiology and medicine at Yale School of Public Health, tells TODAY.com. "What kind of took us off was the pandemic," Ko adds.

Prevention measures implemented to curb COVID-19 were likely effective in preventing norovirus outbreaks, says Grusich, and as restrictions have relaxed, the number of outbreaks has returned to levels similar to pre-pandemic years.

“This is certainly higher than what we experienced during COVID, but it’s in the order of what we saw prior to the pandemic, give or take there’s variation year to year,” says Ko.

While the jump in the norovirus test positivity is concerning, Ko says, it is not unexpected. “I would be surprised if this year’s peak is worse than last year’s. ... I think we’re going to be starting to get back to the epidemiological pattern that we normally see,” says Ko.

The same virus strains that circulate in the U.S. are often what surges in Europe, says Alhassani. Although cases are starting to level off in England, Ko notes, it's still unclear whether this record surge is a harbinger of how severe norovirus will get in the U.S.

How does norovirus spread?

Norovirus is transmitted primarily "when bacteria or viruses shed in stool ends up on our hands and surfaces and then eventually ends up in our mouth and we ingest it and get infected," says Alhassani.

Norovirus may be transmitted directly from an infected person or from contaminated surfaces, objects, foods or drinks.

"Anybody who is in close contact with someone who has an active infection with norovirus is at high risk of getting it," says Alhassani. Norovirus can spread through activities like caring for an infected person, changing diapers or sharing utensils.

"Norovirus is so infectious that even if somebody throws up and there's droplets of vomit aerosolized in the air, that can actually cause infection," Alhassani adds.

It takes a very small number of virus particles to transmit the disease, says Ko, which is why norovirus causes so many explosive outbreaks. Per the CDC, less than 100 norovirus particles can make you sick, and infected people typically shed billions of particles.

Most people are infectious from the time symptoms begin until about two or three days after symptoms resolve, Ko says, but some people can remain contagious or up to two weeks after recovery.

Outbreaks often occur in schools, day cares, nursing homes and cruise ships, the experts note.

What are the symptoms of norovirus?

The most common symptoms of norovirus are vomiting, nausea, diarrhea and abdominal pain, says Ko. Other possible symptoms include a headache, body aches and a low-grade fever. Norovirus symptoms usually develop within 12 to 48 hours after exposure, per the CDC.

"Norovirus ... will just last a few days," says Ostrosky. "For the majority of the population, it’s going to be just a nuisance."

Those at higher risk of developing severe or prolonged symptoms include babies, the elderly and the immunocompromised, says Ostrosky. If symptoms transition into chronic diarrhea and weight loss, this can lead to complications like dehydration or poor absorption of medications, he adds.

What is the treatment for norovirus?

"There's actually no specific treatment or antiviral for norovirus," says Ostrosky. Hydration is key to replenish fluids lost from vomiting and diarrhea, the experts note, which means drinking plenty of water, Pedialyte or sports drinks.

"Then it's just eating bland foods and trying to let it pass through the body, which usually takes like one to three days," says Alhassani, adding that over-the-counter anti-nausea medicine and pain relievers may also be used to ease symptoms.

“The vast majority of people can be managed at home and, in fact, should be isolated at home until they’re improving, given how contagious norovirus can be,” says Ostrosky.

However, it's important to watch for signs of severe dehydration and to contact a health care provider if these occur, the experts note. These include dry mouth, decreased urination, dizziness and, in children specifically, crying without tears, fussiness or unusual sleepiness, per the CDC.

Children under 1, people who are immunocompromised, or those with prolonged or severe symptoms should also be seen by a physician, says Alhassani. “While it infects many people, (norovirus) tends to not send as many people into the hospital and certainly the ICU,” he adds.

A person can be infected with norovirus multiple times in their lifetime. After recovering, you may possibly develop some short-term immunity, says Ko, but it won't be robust and it wanes quickly.

“It's only partial immunity ... because there are different types of norovirus, and being exposed to one doesn’t give you complete protection to another,” he says.

To prevent norovirus infection, don't make this mistake

There's no vaccine against norovirus, says Ko, but there are steps you can take to prevent infection and transmission.

Hand hygiene is extremely important — but the way you clean your hands matters, Ostrosky notes, and it has to be with soap and water. Hand sanitizer does not work against norovirus.

“Norovirus is one of the few viruses that doesn’t get deactivated by alcohol. You actually need to use soap and water to physically destroy it and remove it from your hands,” says Ostrosky.

Wash your hands after using the restroom, before eating or cooking and after caring for someone with norovirus.

When cleaning surfaces or objects that may be contaminated with norovirus, Ostrosky suggests using a high-level disinfectant like bleach.

If you or your child are sick with norovirus, isolate to prevent the virus from spreading within the household, says Alhassani. Anyone sick with norovirus should stay home until they feel better. “Avoid food preparation until at least 48 hours after symptoms stop,” says Grusich.

"We can expect to continue seeing more viral illnesses, both respiratory and gastrointestinal, in this post-COVID era we're sort of approaching," says Ostrosky, adding that the basics of hand-washing, isolation and respiratory etiquette can go a long way.

This article was originally published on TODAY.com