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By Erica Zurek, Minnesota Public Radio News

Snow covers the cross-country ski trails, and the skate deck is freshly groomed. But instead of pushing off and gliding away, your stomach gurgles and a wave of nausea takes hold, signaling a sudden urge to throw up.

Norovirus, one of winter’s grossest viruses, is spreading at high rates in Minnesota and many other areas of the U.S.

Sometimes referred to as the “winter vomiting bug,” noroviruses are the leading cause of foodborne illness in the state for people of all ages. Unfortunately, they cannot be prevented by fresh air or skiing.

This year’s norovirus case uptick has been more ruthless than usual. The Minnesota Department of Health reported 76 outbreaks of norovirus in December, three to four times the usual number for this time of year. Each outbreak represented more than 1,500 individual cases.

There have already been 26 outbreaks in January alone.

Health care providers are not required by law to report norovirus cases to the state health department, so the data reflect estimates from schools, long-term care facilities and day cares.

If anything, these estimates significantly undercount the amount of norovirus cases in Minnesota, according to the health department.

Tools for detecting pathogens have improved and clinicians are doing a better job of testing for them, one possible explanation for why case numbers are higher this year.

“In the past when you got tested it might take several days for a test to come back as positive for a particular pathogen because there was a whole process of actually culturing the pathogen, and it took quite a bit of time,” said Melanie Firestone, assistant professor of public health at the University of Minnesota. “Now, with these rapid tests, we can test for a wide range of different pathogens all at once and have the results very quickly.”

Firestone said in addition to testing, government agencies have invested in better tracking methods and can identify causes of disease. This heightened awareness means illnesses previously called a generic “stomach bug” are now being diagnosed as norovirus.

The many different types of noroviruses make it difficult for humans to build effective immunity against them. Currently, there is no approved vaccine.

Symptoms of norovirus infection develop 12 to 48 hours following exposure to the virus and can include short intense episodes of vomiting and diarrhea, sometimes accompanied by headaches and a low-grade fever or chills. The illness usually resolves on its own after a couple of days.

The virus tends to spread in the colder months from November to April, but health professionals are not sure why norovirus cases, like with the flu and RSV, are higher in winter. The best guess, said Dr. Susan Kline, a professor at the University of Minnesota’s Medical School and infectious disease physician with M Health Fairview, is simply that people spend more time together indoors in winter.

“It is not just the seasonality, but person-to-person transmission is important with all these infections,” she said. “When you have more humans close together in an indoor space viral infections are more likely to spread.”

The gastrointestinal norovirus’s main mode of transmission is the fecal-oral route, meaning infected individuals shed traces of the virus through their stool and vomit which can then contaminate food, drinks and surfaces. When someone inadvertently touches or eats something that is contaminated, the virus spreads.

People shed billions of virus particles while infected with norovirus, and Kline said the illness is extremely contagious because it is estimated that only a small number of particles are needed for a person to infect someone else.

A single gram of feces from an infected individual can contain 10 billion infectious doses of norovirus, according to the Minnesota Department of Health.

“The virus is transmitted relatively readily from that sort of low-level fecal contamination into someone else’s mouth and that sets up the infection,” Kline said, reinforcing norovirus’s status as a gross virus champion. “It can also spread through vomit and be transmitted through airborne droplets which then could be inhaled.”

On top of being highly contagious, norovirus is difficult to completely eradicate from the environment. Health officials recommend disinfecting surfaces with bleach-based cleaning products immediately after coming in contact with vomit or diarrhea.

Alcohol-based sanitizer does not eliminate norovirus, but most illnesses and outbreaks can be prevented by washing hands with soap and water before preparing or eating foods, and after using the bathroom or changing diapers.

People with norovirus are contagious from the moment they start feeling sick until at least three days after they recover. The American Medical Association says some people can shed the virus for up to two weeks after their symptoms are gone.

“So, as a precautionary measure, if you are ill with vomiting and diarrhea, it is better to stay home,” Kline said.