Newsweek has created a map showing where in the country norovirus is surging as reported cases of the stomach bug continue to increase.

There were 91 outbreaks of norovirus, also known as stomach flu , reported during the week of December 5 (the most recent official data). This was up from 69 outbreaks during the last week of November.

It is also much higher than the maximum of 65 reported on average in the same week of December over the past 12 years, according to data from the U.S. Centers for Disease Control and Prevention.

Norovirus Outbreaks By State—Map

California had the most number of cases (43), followed by Florida (24), New York (21) and Virginia (20), between September 1, 2023, and October 21 this year, according to the CDC database CalciNet Data.

Other states with high figures included Ohio, Wisconsin and Minnesota (18 each), Rhode Island (16), Oregon (13), West Virginia and Michigan (11 each).

Meanwhile, states in which between one and four cases were reported in that time included Arizona, Utah and New Mexico (two each) and Kansas, Illinois, Washington, Wyoming and Idaho (one each).

In some states zero cases were reported, including North Dakota, South Dakota, Oklahoma, Arkansas, Georgia, New Jersey, Maine and New Hampshire.

Norovirus Outbreaks By Region–Map

The Midwest region had the highest percentage of positive norovirus cases (26 percent of 1,509 tests carried out) in the week ending December 21, according to the CDC's National Respiratory and Enteric Virus Surveillance System.

In the same week, 21.17 percent of 463 tests carried out in the Western region were positive, while 18.90 percent of 418 tests done in the Southern region were positive and 12 percent of the 308 tests carried out in the Northeast region were positive.

What Is Norovirus?

Norovirus is the leading cause of foodborne illness in the U.S., responsible for nearly 58 percent of foodborne infections annually.

Dr. William Schaffner, an infectious disease professor at Vanderbilt University School of Medicine in Nashville, Tennessee, told Newsweek on Sunday that "the illness is brief, but fierce."

He classified the typical symptom development as beginning "with a sudden explosive vomiting episode which soon is followed by more vomiting and often diarrhea."

Typically, most people recover within one to three days , with the exception of the elderly, immunocompromised people, and some children, he said.

"Dehydration is of special concern for the very young and older frail people who may require hospitalization for rehydration," Schaffner said.

While outbreaks most often occur from November to April, Dr. Jatin Vyas, an infectious disease professor at Columbia University Medical School in New York City, previously told Newsweek that, given "the majority of illness are related to our food supply," infectious disease experts have begun to "think of norovirus as kind of a year-round process, and there's just kind of blips and valleys."

The CDC has identified leafy vegetables, fresh fruits, and shellfish as a common source of norovirus outbreaks.