Gregory Hartl, of the World Health Organization, said the strain of the virus seen in Mexico is worrisome because it has mutated from older strains.
"Any time that there is a virus which changes ... it means perhaps the immunities the human body has built up to dealing with influenza might not be adjusted well enough to dealing with this new virus," Hartl said. He said that, in Mexico, otherwise young, healthy people have been hit by the virus -- "one of the pieces of the puzzle that is worrying us," he said.
Mexico City has closed all of its schools and universities until further notice because of the virus.
More than 1,300 people with flu-like symptoms have been admitted to hospitals in Mexico, and officials are trying to determine how many of them have swine flu, said Jose Cordova Villalobos, the country's health minister.
The H1N1 strain of swine flu is usually associated with pigs. When the flu spreads person-to-person, instead of from animals to humans, it can continue to mutate, making it a tougher strain that is harder to treat or fight off.
The United States had not issued any travel warnings or quarantines.
But US Airways said Saturday night it would allow passengers to change plans if they wanted to because of the outbreak.
Airline spokeswoman Michelle Mohr said it was not asking people not to travel to Mexico, but wanted to "give them that flexibility" if "they don't feel comfortable."
The Canadian Public Health Agency had issued a travel health notice, saying, "The Public Health Agency of Canada is tracking clusters of severe respiratory illness with deaths in Mexico." Symptoms of swine flu include fever, lethargy, lack of appetite, coughing, runny nose, sore throat, nausea, vomiting and diarrhea, the CDC said.
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