Canada may have fought off Sars, but now public health officials across the country are getting ready to battle another potentially deadly virus that is new to north America.
Fighting the spread of the West Nile virus means doing battle with mosquitoes, which are as much a part of the Canadian summer as sunshine and cold beer.
The virus is spread to people through mosquitoes, although it does not only affect humans. It is an avian virus, passed on by mosquitoes that feed on the blood of both birds and people.
However, the good news is that an estimated 80% of people infected with the virus feel no symptoms other, perhaps, than an itchy red spot where they were bitten. Only 20% develop a rash or fever.
The bad news is that about 1% become really sick, suffering from meningitis, encephalitis and polio-like paralysis or tremors similar to those associated with Parkinson's disease.
The West Nile virus was first isolated in Uganda in 1937 and, while it is common in Africa and the Middle East, it first spread to north America in 1999, when 69 people in New York became infected.
Last summer, it spread to Canada, and an outbreak in Ontario, the country's most populous province, was worse that it had originally appeared.
Public heath officials had reassured people that only the elderly and those with compromised immune systems were in danger. In fact, young, healthy Canadians were hit badly by the virus. A year later, some are still in wheelchairs.
The results of one recent study showed that 1,000 people in Ontario needed medical attention for the West Nile virus last summer, and 12 had died.
So far, most the cases have been in urban areas, seeming to suggest that the biggest culprit is an urban species of mosquito, Culex Pipiens, that favours storm sewers or drainage ditches and can survive the winter in cities like Toronto. "When the spring comes, she's already in town and ready to bite," says Mario Boisvert, a Montreal biologist.
Canada is home to dozens of species of mosquitoes, and it is not known how many of them carry the virus.
The north American strain of West Nile appears to be more virulent than elsewhere. Experts say that it is killing more birds and making more people ill, but that could be because north American people and wildlife alike have not built up immunity to it.
Across the country, city authorities are considering spraying insecticides that will kill mosquito larvae into drainage ditches and sewers. They are not ruling out the possibility of spraying potentially toxic chemicals in some neighbourhoods in order to kill fully-developed mosquitoes, a move opposed by environmentalists.
The odds of catching the virus remain very low, but public health officials say that people who want to protect themselves from it should stay inside at dawn and dusk, wear long sleeves and trousers, and use insect repellent.
Canadians are also being urged to dispose of any containers of water - even small gardening pots or old coffee cups - in which mosquitoes could breed.
Ontario, and Toronto is particular, is only now emerging from the shadow of Sars, which killed more than 20 people. Last week, the World Health Organisation removed Toronto from its list of Sars-affected areas, and it lifted devastating travel advice, warning people not to travel to Canada's largest city, on April 29.
Now the West Nile virus must be dealt with, and nobody can predict how hard it will hit. Winters are long and cold in much of Canada, and most Canadians look forward to the warmer months as a respite from snow and ice. Now, summer may never be the same again.
