Health officials are investigating after a west Toronto school had 250 students absent with flu-like symptoms.

St. Gregory Catholic School, near Kipling Avenue and Rathburn Road in Etobicoke, posted a letter on its website Monday saying "a significant number of students" were absent after getting sick. An "additional number" of students left school later that day, the note from the principal said.

Dr. Michael Finkelstein, Toronto's associate medical officer of health, said he believes a norovirus may be to blame.

Norovirus — which can spread from person to person, through tainted food or in water contaminated by feces — is a common cause of gastroenteritis but is rarely fatal. Finkelstein said the virus can spread quickly.

"People get vomiting, or they have diarrhea, and they contaminate the washroom. And so people come into the washroom afterwards, they touch the taps or the countertops and then they touch their mouth or their nose," he said.

The Catholic District School Board contacted Toronto Public Health — which is now investigating — and has stepped up efforts to clean and sanitize the school premises.

Emmy Milne, a spokeswoman for the school board, said about 250 students were absent Monday. About 150 students were absent Tuesday, she said.

The cause of the absences "was described to us as what appeared to be a stomach virus," Milne said.

Milne couldn't say what could have caused the spate of illnesses. She said the school held a Halloween dance on Friday for middle school students, "which would have brought ... a fair number of students together in close proximity."

She couldn't say whether food was provided at the dance.

A number of staff members with similar symptoms were also absent from the school, which has over 600 students in junior kindergarten to Grade 8.

A principal's note sent to parents urges them to keep ill children at home and not to send them back to school until 48 hours after their symptoms have gone.

Toronto Public Health advised those worried about contracting a virus to wash their hands frequently and to avoid contact with surfaces that could be contaminated.