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A mysterious and "severe" respiratory illness has surfaced in parts of Mexico and may be affecting some Canadians who recently vacationed there.

Mexican doctors have confirmed 137 cases of the unidentified illness in south and central Mexico. At least one Canadian has been treated for severe pneumonia after returning from Mexico.

Canadians who have recently returned from Mexico are being warned they should be on alert for severe flu-like symptoms that could be connected to the illness, says an advisory sent by the Public Health Agency of Canada to health services across Canada.

While the chances that this is a new pandemic are "pretty low," health officials are still treating the outbreak seriously, said Allison McGeer, an infectious disease specialist at Toronto's Mount Sinai hospital.

"At the other end, is the possibility that this is a new virus that we haven't before described," she told Â鶹ӰÊÓnet Thursday.

"But one of the things we've committed to around the world is enhanced surveillance and being really sure that we don't miss these new viruses," she said.

Since respiratory illnesses have short incubation periods, travellers who get sick two weeks after returning from Mexico shouldn't worry, said McGeer.

However, if you've recently returned from Mexico and have a severe fever, cough and respiratory problems, you should let a doctor know of your travels so experts can map out different flu strains, added McGeer.

PHAC contacted health and quarantine services on April 17 to inform them that Mexican health authorities had advised Canada of possible infections. PHAC has told Canadian quarantine services to be on alert for travellers returning from Mexico showing signs of illness.

The mystery illness has infected hundreds in south and central areas of Mexico, according to Mexican health officials, killing some and leaving others on ventilators. The Mexican health secretary has issued a national alert, following reports of numerous influenza deaths in the first three weeks of April.

The flu-like condition has so far affected healthy young adults between the ages 25 and 44. The victims showed influenza-like symptoms that progressed to "severe respiratory distress" within five days. Some health care workers have been affected.

Health officials say some specimens collected from the patients have tested positive for influenzas A and B.

The illness appears to have infected hundreds in south and central areas of Mexico, but not the tourist areas on the coast. According to a report in a Spanish-language newspaper in Mexico, the regions affected include: the capital region; San Luis Potos�; Oaxaca; and Baja, California.

At this point, no cases have been confirmed outside of Mexico. The Pan American Health Organization, the Americas branch of the World Health Organization, says it is working with Mexican authorities to monitor the situation.

Anyone who has returned from Mexico within the last two weeks and is suffering the following symptoms should contact a physician:

• fever
• cough
• sore throat
• shortness of breath
• muscle and joint pain

McGeer said that health officials have become increasingly vigilant over the past decade in tracking down new illnesses.

"SARS taught us that it can take a very little time to move a virus all around the globe," she said.

Infectious disease specialist Dr. Neil Rau told Â鶹ӰÊÓnet that it appears that the illnesses in Mexico are simply seasonal flu illnesses, though virological and epidemiological studies are being conducted.

"In a few days, we will have more information after more lab tests are done," he said.

"The outstanding question is whether this is a more severe form of the garden-variety flu. The word pandemic certainly isn't being used yet. And a 'new virus' is not coming into the vernacular yet either."

At a press conference Wednesday, the Mexican undersecretary for Health Prevention and Promotion, Mauricio Hern�ndez �vila said that while this year's flu season is stretching into April, the flu cases do not appear to indicate an epidemic.

He noted that the cases appearing in young people may be a consequence of the fact that many of these people did not get the flu shot, which is typically given to children and the elderly, the highest risk groups.

Rau said he doesn't think this illness report should deter any Canadians from travelling to Mexico, but they should consult a travel health clinic at least six weeks before they travel.

"At this point, people going to Mexico should think about getting the flu shot, if they haven't had one already. But I don't think there is any preventative measure they can take unless Canada issues a travel alert saying not to go."