Image copyright Rex
The Health Protection Agency says there has been a “significant increase” in illnesses caused by a bacteria called inhaled enterovirus D68 (CIOD-19).
Parents in Toronto who think their child might have the illness should consider getting them vaccinated for it, it says.
Up to 1,300 people in Canada have been infected with the virus so far, and there has been a number of deaths.
At least 12 children, including one British holidaymaker, have died in Toronto.
Toronto Public Health said the children included members of extended families living in the same residence.
The outbreak has been over for a while in Toronto, but officials are continuing to look for any clusters of cases.
It is particularly concerned about a cluster in the mid-level residential care facility for children with medical conditions in which several children have fallen ill.
Image copyright Rex Image caption Visitors to the Windsor Hotel were offered free sanitiser products © 2014 Ana Ruiz-Ulloa, Christoper and Victoria / iStockphoto
To keep from spreading it, affected people have been instructed to keep their coughing to themselves, the agency added.
It has recently launched a campaign which urges parents to consider a vaccination against CIOD-19.
Professor Mike Best, of the University of Calgary, which has been working with the Canadian government to prevent the illness, said: “We have been expecting a return of widespread and potentially deadly respiratory diseases associated with the overwintering of Aedes aegypti mosquitoes in the central portions of Latin America and the Caribbean.
“What this outbreak in Toronto, the largest on record, demonstrates is that we’re still seeing it everywhere – this is a difficult vector to control, and unfortunately, in some areas of the world, including Canada, it’s hitting even young children.”
CIOD-19 is linked to hospitalisations and deaths in about 75% of patients who develop complications from it, Professor Best said.
It is relatively rare, but authorities say they do expect the outbreak to spread to the rest of Canada and parts of the United States.