TORONTO - Canadians should stay vigilant about fending off mosquitoes in the coming weeks, health officials said after an Ontario woman's death was linked to the West Nile virus.

With September fast approaching, some people are letting their guard down when it comes to protecting themselves against bug bites, said Dr. Monir Taha, associate medical officer of health for Halton region, southwest of Toronto.

"But looking at past years, August and September are months where the virus is fairly active and that's when we see human cases," he said Thursday.

The warning came after a Burlington, Ont., woman believed to have contracted the West Nile virus died last week.

Tests are underway to confirm whether the woman, who was in her 70s, had the virus, but Taha said it's the most likely diagnosis.

"The fact that we've had one reported human case means there's probably more," though people may not experience any symptoms, he said.

Most people who have West Nile -- about 80 per cent -- won't feel ill at all, while another 20 per cent show flu-like symptoms such as fever, headache, nausea and sometimes a rash, he said.

Fewer than one per cent develop serious and potentially deadly conditions such as meningitis, and even fewer -- roughly one in a thousand -- die as a result of the infection.

Older people and those with weakened immune systems are far more vulnerable to the virus, which is transmitted by mosquitoes that bite infected birds, Taha said.

"The risk for serious illness increases with age," he said.

"In past years, we had parents running around worried about their kids when they should have been worried about themselves or their own parents."

The virus first appeared in Canadians in 2002 and by the following year, more than 1,400 cases were reported, according to the Public Health Agency of Canada.

There's been a steady decline ever since.

In Ontario, the virus hit its peak in 2002 with 394 cases, said Andrew Morrison, spokesman for the Ministry of Health and Long-Term Care.

This year, only two people have tested positive for the virus, he said.

That's in spite of a boom in the number of mosquitoes carrying it, he said.

Officials monitoring the disease have found it in more than 130 "pools" of mosquitoes across the province this year, compared with 57 in 2010, he said.

Experts say the spring's wet weather is likely to blame because it creates optimal conditions for the insects to breed.

While that doesn't guarantee there will be more cases, "it's up to people to take precautions," said Dr. Brian Schwartz of Public Health Ontario, an arms-length government agency that promotes health in the province.

Keeping mosquitoes at bay goes a long way to reducing the risk of West Nile, he said.

The easiest way is to simply avoid going outside at dawn and dusk, both prime times for bug bites, he said.

"If you do go out, wear protective clothing," such as pants and long-sleeved shirts, preferably in light colours, he said.

Insect repellents containing the chemical DEET also help, he said.

Taha said getting rid of stagnant water -- like a birdbath or rainwater collected in an upturned garbage lid -- limits the areas where mosquitoes can breed.