The US is seeing a rise in the norovirus informally called the stomach flu or stomach bug, according to the American Centers for Disease Control and Prevention (CDC), with positive tests for the contagious illness peaking at 16% in January.

The rise in infections spans the US, according to the healthcare agency, with infection rates not seen since last spring.

The illness is highly infectious, spreading easily through close contact with others, touching surfaces or consuming contaminated food or beverages. Those infected can shed billions of particles with the illness, and only a few particles can make a person sick.

While immunity upon infection is possible, it is unclear how long it lasts, making repeat infections possible.

Symptoms of the illness include nausea, diarrhea and stomach pain as the virus causes inflammation in the intestines, the CDC said. There is no treatment for the illness, though symptoms – while unpleasant – are usually mild and clear up in a few days. Doctors recommend drinking plenty of liquids when dealing with an infection.

Health experts emphasize frequent hand washing with warm water and soap as an important measure to prevent norovirus. Hand sanitizer is not as effective against the virus as hand washing.

Regions across the country are seeing outbreaks, particularly in schools. Recent outbreaks have been reported in and around Washington DC, Detroit and Las Vegas, among other places.

Though norovirus infections are increasing around the country, the CDC says contagion levels are on par with what was seen before the Covid-19 pandemic as activity restrictions related to that crisis have continued to ease. Norovirus usually peaks between November and April of any given year.

“Norovirus outbreaks and reported cases from both state health departments and clinical laboratories are increasing but remain within the expected range for this time of year,” a spokesperson for the CDC, Kate Grusich, told NBC News.

“Prevention measures implemented during the Covid-19 pandemic were likely effective in preventing norovirus outbreaks. As pandemic restrictions have relaxed, the number of norovirus outbreaks has returned to levels similar to pre-pandemic years.”

In September, the CDC released a report that showed that the number of norovirus outbreaks from August 2021 to July 2022 was three times more than outbreaks seen in the previous year.

Nearly three years after Covid-19 saw widespread lockdown measures and mask mandates, such policies are largely nonexistent. Hospitals are some of the only places that require masks, and some – like ones in New York state – are starting to drop their masking policies.

Experts have pointed to the seemingly bad flu season and spread of respiratory syncytial virus – or RSV – this winter as other illnesses that have seen a resurgence since the rescinding of Covid-19 measures that also kept those other illnesses in check. The CDC has estimated that this flu season has seen at least 25m illnesses and 18,000 deaths. Covid-19 is also still circulating, with about 280,000 cases and 3,100 deaths being reported weekly.