Four cases of swine flu have now been reported in Ontario, bringing the total number of Canadian cases to 13. All of the Ontario cases were found in the Toronto area and have been linked to Mexican travel.

"All cases are the same strain of the human-swine influenza that has been found in the United States and Mexico, and all of the confirmed cases in Canada so far have been mild," Canada's chief public officer of health David Butler-Jones told reporters Tuesday.

Ontario's acting chief medical officer, Dr. David Williams, said he believes the province will see more cases in the future.

"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," he said in a separate news conference.

Also on Tuesday, health authorities in Alberta confirmed two mild cases in the province: one in Calgary and the other in the northern part of the province.

Alberta's chief medical officer Dr. Andre Corriveau said both patients were men who contracted the virus during recent travel to Mexico. Neither man's illness was severe enough that he had to be hospitalized.

"These cases show that our monitoring and surveillance system is working as it should," Corriveau said in a news release.

Other confirmed swine flu cases include three in B.C. and four at King's Edgehill School in Windsor, N.S.

King's Edgehill Headmaster Joseph Seagram told Canada AM on Tuesday that three of the four students have now been given a clean bill of health. The fourth is "recovering nicely and we expect he will be released today."

So far, only Mexico has seen severe disease related to the virus, but Butler-Jones said that could change.

Meanwhile, the Public Health Agency of Canada is advising against all non-essential travel to Mexico, due to the swine flu outbreak there that has caused the deaths of 152 people.

"As of April 27, 2009, travellers from Canada are recommended to postpone elective or non-essential travel to Mexico until further notice," stated the advisory.

The U.S., France, Britain and Germany are also warning against unnecessary travel to Mexico.

Ottawa recommends that those who must travel to Mexico do the following to minimize risk:

• Wash hands frequently with soap under warm running water.
• If water isn't available, use an alcohol-based gel hand-sanitizer.
• Cover your mouth and nose when you sneeze, and wash your hands afterwards.
• Avoid contact with people who appear to have respiratory illness.
• Monitor Canada's and Mexico's public health agency websites.
• Get vaccinated against seasonal flu.

Following the travel advisory, Canada's biggest travel companies cancelled trips to Mexico, including Transat AT, which will bring all employees and customers back to Canada.

The company said that flights to Mexico will be cancelled until at least June 1 and will impact people who booked trips with Air Transat, Transat Holidays, Nolitours, Vacances Transat and Look Voyages.

Air Canada and Air Canada Vacations also announced Tuesday they will suspend all operations to Cancun, Cozumel and Puerto Vallarta, Mexico, until June 1. The airline said, however, it will keep its flights to Mexico City going.

Air Canada Vacations customers booked for Cancun, Cozumel and Puerto Vallarta may rebook their holiday with no change fees to any destination or resort offered by the company.

Officials monitor illness globally

Dr. Donald Low, an infection disease specialist at Mt. Sinai Hospital in Toronto, says he doesn't believe there are any parts of Mexico that are swine flu virus-free.

"We really have to appreciate now that this has spread throughout all of Mexico. We're hearing about cases in all four corners of Mexico. We're hearing about people coming back from all over Mexico with this disease," he told CTV Newsnet.

"So what it means is there are probably hundreds of thousands of people who have been infected with this and we're only seeing the tip of the iceberg by identifying only the very sick. That's why it looks like a very severe influenza in Mexico. But in actual fact, it may have a death rate that's similar to seasonal influenza.

"We're not talking about avian influenza or SARS where we saw a death rate of 10 per cent."

The World Health Organization is monitoring the global infection rate. On Monday, they raised the pandemic alert level from a Phase 3 to Phase 4 level, marking the increasing severity of the outbreak.

The Phase 4 classification -- the first since 1999 --means human-to-human transmission is taking place.

The World Health Organization says it may bump the pandemic alert to Phase 5 if it finds evidence the swine flu viruses is passing beyond second generation spread. Most cases to date outside of Mexico are in people who became infected in Mexico and travelled home.

If the WHO sees travellers infecting family members, who infect work colleagues and so on, that would signal that the swine flu virus is well adapted to human spread and poses an even greater pandemic threat.

WHO spokesperson Paul Garwood said the organization is respecting individual countries' handling of the outbreak and said travellers should be cautious and follow their government's travel recommendation.

"We feel now that the virus has been found in several countries it's difficult to contain, so we need to focus on mitigation efforts," Garwood told CTV's Canada AM on Tuesday.