Letters have gone home to parents at Thornhill Woods Public School in Thornhill, after an outbreak of what appears to be norovirus.
Dozens of children have been sent home after getting the gastrointestinal bug.
The school is in the Bathurst and Highway 7 area.
York Region Public Health says they're still waiting on lab results to confirm what has caused the outbreak.
However, Marjolyn Pritchard, Director of Infectious Disease Control, says norovirus is common this time of year.
"We've seen it elsewhere in the community so it's not an unusual suggestion that it could be norovirus," she told NEWSTALK 1010.
Norovirus is highly contagious and symptoms include vomiting, diarrhoea and stomach cramps.
The school is asking parents to keep their kids home until they are free of symptoms.
The building is being cleaned and disinfected, top to bottom next weekend.