One in four Americans with a bad stomach has norovirus as cases rise… how do you know if you’re one of them

One in four Americans tested for stomach viruses has the ultra-contagious norovirus, according to new data.

The CDC reported Monday that 28 percent of tests for norovirus, the most common foodborne illness in America, were positive in the first week of January.

This amounts to just over 820 positive infections, a fourfold increase in the week of December 7.

However, the actual number of infected Americans is likely much higher, as most people are not tested and the disease affects millions of people every year.

Cases of norovirus have doubled compared to this time last year, when just over one in 10 tests were positive.

And data from the CDC shows that norovirus is experiencing its sharpest increase since before the pandemic, as test positivity rates ranged from one to 16 percent between January 2019 and January 2024.

Additionally, rates are highest in Midwestern states including Missouri, Illinois, Minnesota, Michigan and Ohio, with one in three tests coming back positive.

The numbers come as the US faces a quad-demic of winter illnesses, prompting some states to bring back mask mandates.

Norovirus is the most common foodborne illness in the US, infecting as many as 21 million Americans annually. It causes diarrhea, vomiting, nausea and stomach pain

Norovirus infects about 21 million Americans annually and sends about 2 million to the doctor’s office or emergency care.

It causes severe diarrhea and vomiting, which can lead to life-threatening dehydration. It spreads through contact with germs from an infected person’s vomit or feces, contaminated food, shared utensils, or surfaces they have touched.

Although most people recover within a few days, the virus kills about 900 people every year, mostly adults 65 and older.

Outbreaks are most common in winter due to an increase in the number of people gathering in groups for holiday festivities, traveling and staying warm indoors, where pathogens can easily spread.

Symptoms of norovirus – nausea, vomiting and diarrhea – occur quickly, within 12 to 48 hours after exposure.

Doctors say the disease can be prevented by washing your hands thoroughly with soap and water for at least 20 seconds, washing produce before eating it, disinfecting countertops, avoiding people who are contaminated, and cooking seafood at at least 145 degrees.

The latest numbers reported to the CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) show that for the week of January 4, according to the most recent data available, 28 percent of the 2,960 tests conducted for norovirus tested positive were.

This equates to approximately 828 cases found that week.

The number of cases has risen sharply since August. During the week of December 28, one in four tests were positive, a total of about 780.

The week of November 30, just after Thanksgiving, 16 percent of cases were positive, which amounts to just over 500.

Meanwhile, by early August there were only 200 cases, a quarter of the cases reported in January 2025.

The lowest test positivity rate in the NREVSS database was 0.6 percent in November 2020, which amounted to approximately 11 positive infections.

This could have been explained by the fact that social distancing around that time reduced the risk of diseases such as norovirus that emerged during the holidays.

The above graph shows the increasing test positivity rate for Region 2

The map above shows which states fit into the U.S. Census Regions into which the CDC has divided norovirus infections

According to the CDC data, states in what the census designates Region 2 had the highest test positivity rate for norovirus.

In these states, about 30 percent of the 1,673 tests were positive, which amounts to 500.

Region 2 includes Missouri, Illinois, Indiana, Ohio, Kansas, Iowa, Nebraska, North Dakota, South Dakota, Minnesota, Wisconsin and Michigan.

Region 3 had the lowest test positivity rate: 14 percent of 501 tests. This amounts to approximately 70 cases.

States in Region 3 are Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, North Carolina, South Carolina, Tennessee, Kentucky, West Virginia and Virginia.

Some scientists have speculated that this year’s surge of norovirus cases could be caused by a new strain against which people have no pre-existing immunity.

Lee-Ann Jaykus, a microbiologist at North Carolina State University who leads NoroCORE, a multi-federal food virology collaboration, said that while the CDC’s genetic analyzes of circulating noroviruses have not revealed significant changes this season , the potential emergence of a new species is a major concern for many experts.

She told me Vox: ‘I can guarantee you that my colleagues at the CDC are doing a lot of sequencing now to find out if there is a new strain, but it is still too early to say if there is one.’