Norovirus cases are on the rise. Here's what you need to know about this nasty bug

For Edward Chou, a first-year biochemistry student at the University of Guelph, the symptoms began shortly after lunch on Monday. 

"I just felt something in my stomach and I just started throwing up and I had to go to the toilet multiple times," he said. "It was really bad."

The University of Guelph is responding to an outbreak of a self-reported stomach illness that has affected more than 150 people. The outbreak is part of a rising trend in stomach bugs causing vomiting and diarrhea.

The Public Health Agency of Canada confirmed in an email that it is seeing higher-than-expected case counts of norovirus. Health officials in the U.S. and U.K. have also reported a rise in norovirus cases.

Testing has not yet confirmed if the University of Guelph's outbreak of gastroenteritis is due to norovirus, but a Wellington-Dufferin-Guelph Public Health spokesperson told CBC that symptoms are consistent with norovirus infection. 

The school has stepped up its cleaning protocols, especially in washrooms and common areas, said Melinda Scott, vice-provost of student affairs at the University of Guelph.

Dr. Zain Chagla, an infectious disease specialist and associate professor at McMaster University, says seasonal norovirus case counts are on the rise because of winter weather, but also because people are no longer engaged in measures like physical distancing that were encouraged during the COVID-19 pandemic to prevent the spread of viruses.

"We disrupted patterns, they're now back to normal," Chagla told The Dose host Dr. Brian Goldman. 

Canadian cases of norovirus have risen above the five-year average between 2019 and 2023, according to preliminary data from PHAC's National Enteric Surveillance program. Increases were noted in multiple provinces including Alberta, Ontario, B.C., Nova Scotia, Saskatchewan and P.E.I.

Chagla also offered another explanation for the higher case counts: physicians could be diagnosing it more often as a result of better testing capacity and a post-COVID emphasis on diagnosis to help control infections.

Noroviruses are a highly contagious group of viruses that are responsible for 90 per cent of all epidemic gastroenteritis outbreaks, according to PHAC. 

A 2017 modelling review looking at norovirus's reproductive rate concluded that one person infected with norovirus could infect anywhere between one and seven other people. In comparison, measles — considered one of the most contagious viruses — has an estimated basic reproductive number that ranges between 12 and 18. 

While norovirus is present throughout the year, it is dubbed the "winter vomiting bug" because of its prevalence during colder months. 

Common symptoms of norovirus infection include nausea, stomach pain and cramps, vomiting and diarrhea. Symptoms typically resolve within 12 to 72 hours.