OTTAWA - The number of confirmed cases of swine flu in Canada has grown to 13 as health authorities in Alberta and Ontario reported their first outbreaks of the virus.
Seven new cases were reported on Tuesday: four in Ontario, two in Alberta, and one in British Columbia.
The new cases, like Canada's previous ones, are all linked to people who have been to Mexico, or travellers who have infected family and friends.
Canadian scientists at the National Microbiology Laboratory in Winnipeg, which helped identify the new flu virus last week, are slowly learning more about the Mexican mystery bug.
It appears to have an incubation period of between two and seven days.
There's also evidence it can spread to a so-called second and third generation, beyond the person initially infected, as the virus infects a second person who then gives it to a third person.
The World Health Organization says it may raise the pandemic alert to Phase 5 on its six-point scale - Phase 6 being a full pandemic - if it finds evidence the swine flu virus can pass beyond second generation.
Health Minister Leona Aglukkaq said while the new cases reported Tuesday are all mild, the spread of the virus heightens the risk of a pandemic.
"While we are not in a pandemic, it simply means that the risk of one has increased," she told a news conference.
Canada's top medical officer, David Butler-Jones, said it's too soon to know how the virus will spread.
"Nature is always inventive, and it can go in either direction," he said.
"You can have severe presentation in Mexico ... or it could be, as in 1918, where the first round was relatively mild and the next round was much more severe."
He was referring to the Spanish flu pandemic that started in 1918, possibly the deadliest outbreak of all time, which killed an estimated 50 million people worldwide.
Butler-Jones noted that there is no evidence the swine flu can be transmitted through food products. He repeated that the best way to prevent the spread of the flu is to wash hands and cover up coughs and sneezes.
He also advised people to call the public health line at 1-800-454-8302 if they want more information.
Aglukkaq said the government will continue with its six-point plan to deal with the threat, including: surveillance, international co-operation, research on vaccine development, stockpiles of anti-viral medication, health services, and communications.
The minister said she has instructed customs officers to begin "enhanced screening" of air travellers from Mexico.
She also said she spoke to the health ministers for Ontario and Alberta earlier Tuesday and would talk to her B.C. counterpart later in the day.
In Ontario, the acting chief medical officer of health said all four cases there were considered mild and the individuals are recovering at their homes in the regions just outside Toronto.
"Personally, I know we're going to see a lot more cases. It's only a matter of time because of the amount of flow of public that go back and forth to Mexico," Dr. David Williams said.
"It would not be surprising that we have some more confirmed cases."
Alberta's chief medical officer, Dr. Andre Corriveau, confirmed two mild cases of swine flu - one in Calgary and the other in the northern part of the province.
Both individuals are male and contracted the influenza virus during recent travel to Mexico, Corriveau said. Neither man was treated in hospital. One no longer has symptoms, while the other has almost made a full recovery.
He wouldn't identify which community in northern Alberta one man is from, partly for patient privacy reasons and also to prevent any panic in that community.
Neither man has been told to isolate himself at home.
"They were advised to avoid spreading the infection but at this point they've recovered. So, as far as I know, there's no reason to keep them from enjoying normal life," Corriveau said.
B.C. health authorities confirmed an additional case Tuesday, bringing the total there to three. An update posted on the website of the B.C. Centre for Disease Control says there are now a total of three cases of the flu in the province.
The lead epidemiologist at the disease control centre, Dr. Danuta Skowronski, echoed the federal government's prognosis that there may be more serious cases and likely even deaths.
But she also sounded a brighter note, saying most flu viruses are seasonal and don't like the hot weather, even in Mexico.
"Influenza viruses don't like sun, they don't like UV light. There's a reason why influenza viruses circulate during the winter period. That's the preferred climate," Skowronski said.
"So we may actually be entering into a hiatus period."
Nova Scotia had previously reported four cases.
On Monday, the World Health Organization raised its pandemic alert level to Phase 4 from Phase 3 to signal the increasing severity of the situation. But the UN agency said it still does not characterize a swine flu pandemic as inevitable.
Mexico has linked 152 deaths to the virus.
The U.S. has diagnosed 64 cases, Spain one and Scotland two. New Zealand health officials announced Tuesday that 11 cases have been confirmed there.
Canada has joined the list of countries advising against travel to Mexico because of the swine flu outbreak.
The advisory from the Public Health Agency of Canada says any non-essential travel to Mexico should be postponed until further notice.
Experts estimate that roughly 19,000 people travel between Canada and Mexico during an average week in April.
Canada's largest travel company, Montreal-based Transat AT, said it is bringing its customers and employees in Mexico back to Canada and postponing any further flights to the country until at least June 1.
"Transat is following the situation closely and could take additional measures depending on decisions that will be made by the public health authorities and governments concerned," the company said in a release.
The Public Health Agency's website says on average, the common flu sends about 20,000 Canadians to hospital each year. Between 4,000 and 8,000 Canadians can die of influenza and its complications annually, depending on the severity of the season.