A Catholic school in Etobicoke may have experienced an outbreak of Norwalk virus-like disease, a Toronto Public Health official said Tuesday.

Dr. Michael Finkelstein, the city's associate chief medical officer of health, told CTV News that the virus usually causes intense illness, but only lasts a few days.

Norwalk or norovirus problems are often called "stomach flu," although it isn't caused by the flu virus, he said.

The viruses do cause a stomach and intestinal infection called gastroenteritis, which can lead to the sudden onset of vomiting and diarrhea, Finkelstein said.

Noroviruses often show up in the fall and early winter, he said.

St. Gregory Catholic School on Rathburn Road had 200 students absent out of 630 on Monday, said Corrado Maltese, the Catholic board's safety manager.

About 150 students were absent Tuesday, he said.

"Things are much better today. We anticipate that things will be much better tomorrow. In terms of staff absenteeism, it was fine."

Staff and students have been reminded of the importance of frequent and thorough handwashing. Extra custodians have been brought into the school to help with additional cleaning and disinfection, he said.

"It's the first little blip we've had at any school in terms of absenteeism," Maltese said.

Finkelstein said they considered the St. Gregory outbreak to be sizable.

Cleaning and disinfection were important because the norovirus can easily be picked up from contaminated surfaces, he said.

Kelly Baker, a spokesperson for the Toronto District School Board, said she's heard no reports of any similar outbreaks within the public system.