But could this outbreak morph into something more alarming?
"We just don't know yet," said Dr. Mark Metersky, spokesman for the American College of Chest Physicians and a professor of pulmonary and critical care medicine at the University of Connecticut School of Medicine in Farmington. "It's early in the outbreak."
Metersky pointed to a 1976 outbreak of swine flu that erupted at Fort Dix, N.J., caused a scare, but then quickly petered out with one death.
Perhaps the most disturbing trend in the Mexican outbreak is the fact that it is primarily young adults who are dying. That's not typical, experts said.
"It is usually people who are weakest, at extremes of age, very young and very old who succumb to influenza and this is a little bit scary because this is a pattern we saw in the [flu] pandemic in 1918," Metersky said.
On the upside, there's a good chance that swine flu may not stick around for long in northern climes, at least not this year. "This will probably have seasonality similar to the one of the regular flu," Topham said. He pointed out, however, that the regular winter flu is never quite gone, because as it wanes from the Northern Hemisphere, it re-establishes in the Southern one.
U.S. health officials declared a public health emergency Sunday in response to the swine flu outbreak, and the number of confirmed cases nationwide had doubled by Monday to 40. The 20 new cases all came from the New York City high school that had previously reported eight cases of the infectious disease.
Some of the U.S. cases, all of which so far have been mild, involved people who had recently returned from trips to Mexico.
Meanwhile, in Mexico, believed to be the source of the outbreak, authorities continued to take dramatic steps over the weekend -- including suspending public gatherings -- to try to contain the swine flu outbreak.
U.S. health officials have reported that t
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