One in four Americans tested for a stomach virus has ultra-contagious norovirus, according to new data.
The CDC reported Monday that 28 percent of tests for norovirus, the most common foodborne illness in the United States, returned positive results during the first week of January.
This adds up to just over 820 positive infections, a four-fold increase from the week of December 7.
However, the actual number of infected Americans is likely much higher, since most people do not get tested and the disease affects millions each year.
Norovirus cases have doubled compared to last year, when just over one in 10 tests were positive.
And CDC data shows that norovirus is experiencing its sharpest rise since before the pandemic, as test positivity rates ranged from one to 16 percent from January 2019 to January 2024.
Additionally, rates are higher in Midwestern states like Missouri, Illinois, Minnesota, Michigan and Ohio, among others, where one in three tests comes back positive.
The numbers come as the United States faces a quadruple epidemic of winter illnesses, prompting some states to reinstate mask mandates.
The graph above shows the increase in positive norovirus tests from August 2024 to the first week of January 2025.
Norovirus is the most common foodborne illness in the U.S., infecting up to 21 million Americans each year. It causes diarrhea, vomiting, nausea and stomach pain.
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Norovirus infects about 21 million Americans annually and sends about 2 million to their doctor’s office or urgent 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.
While most people recover within a few days, the virus kills about 900 people each year, mostly adults age 65 and older.
Outbreaks are more common in the winter due to an increase in the number of people gathering in groups for Christmas festivities, traveling and staying warm indoors, where pathogens can easily spread.
Norovirus symptoms (nausea, vomiting, and diarrhea) appear quickly, within 12 to 48 hours after exposure.
Doctors say the illness can be avoided by washing your hands thoroughly with soap and water for at least 20 seconds, washing produce before eating it, disinfecting countertops, avoiding people who have been infected, and cooking seafood to at least 145 degrees. .
The latest figures, reported to the CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS), show that for the week of January 4, the latest data available, 28 percent of the 2,960 tests performed for norovirus were positive .
This adds up to approximately 828 cases found that week.
Cases have seen a sharp increase since August. During the week of December 28, one in four tests was positive, for a total of approximately 780.
The week of November 30, just after Thanksgiving, 16 percent of cases were positive, totaling just over 500.
Meanwhile, at the beginning of August there were just 200 cases, a quarter of those reported in January 2025.
The lowest test positivity rate in the NREVSS database was 0.6 percent in November 2020, amounting to about 11 positive infections.
This could have been explained because social distancing at that time reduces the risk of diseases such as norovirus that emerge during the holidays.
The graph above shows the increasing test positivity rate for Region 2
The map above shows which states fit into the US census regions. The CDC has divided norovirus infections into
According to CDC data, states in what the census designates as Region 2 had the highest rate of positive norovirus tests.
In these states, about 30 percent of 1,673 tests were positive, for a total of 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 at 14 percent of 501 tests. This adds up to approximately 70 cases.
Region 3 states 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 spate of norovirus cases could be caused by a new strain to which people have no preexisting immunity.
Lee-Ann Jaykus, a microbiologist at North Carolina State University who directs NoroCORE, a food virology collaboration between multiple federal agencies, said that while genetic analyzes of circulating noroviruses by the CDC have not revealed any significant changes this season, the possible emergence of a new strain is a key concern for many experts.
she said voice: ‘I can guarantee you that my colleagues at the CDC are doing a lot of sequencing now to determine if there is a new strain, but it is too early to say that there is.’