By Lee Carter BBC, Toronto

Canada could soon be facing up to another mystery illness, just as the pneumonia-like Sars virus appears to be becoming manageable.

West Nile virus, a flu-like illness, last year killed 17 people in Canada and 284 people in the United States.

An early mosquito season is fuelling concern that Canada is set for a difficult summer.

The disease, originally from northern Africa and the Middle East, was first detected in North America in 1999 and is spread by the common mosquito.

Canadians are being urged to take precautions

Birds are much more susceptible to West Nile virus than humans so the recent discovery of several dead crows in the Toronto area that have clearly died from the disease has restarted a debate here about how people can protect themselves.

About 80% of people infected with West Nile virus show no symptoms and most of the rest suffer from an illness rather like influenza.

But for some, especially those with a weak immune system, the disease can result in paralysis or death.

A late winter and melting snow has apparently left many pools of stagnant standing water - a perfect breeding ground for mosquitoes.

In Montreal, authorities have begun spreading a biological larvacide to help prevent mosquitoes from leaving the larval stage. Other cities have taken the less sophisticated approach of spraying gallons and gallons of insecticide from the backs of lorries.

Local authorities are urging Canadians to remove all sources of standing water on their properties, avoid going outdoors during peak mosquito feeding times - basically dawn and dusk - and to always use insect repellent outside.

The country's blood agency is also redoubling its efforts to keep the virus out of the blood supply.