Swine Flu Likely to Return to U.S. N...(all in the Southern hemisphere. Experts ...),Health

"You have this risk of an additional virus that could essentially cause two outbreaks at once," Dr. Jon Andrus, of the Pan American Health Organization's headquarters in Washington, D.C., told the wire service.

Two separate flu strains could also mutate into a new strain that is more contagious and dangerous. "We have a concern there might be some sort of reassortment, and that's something we'll be paying special attention to," World Health Organization spokesman Dick Thompson told AP.

In North America, the summer should slow down the spread of swine flu; neither viruses nor bacteria survive well at temperatures above 70 degrees Fahrenheit, explained C. Ed Hsu, an associate professor of public health informatics at the University of Texas School of Health Information Sciences at Houston and associate director of health informatics at the Center for Biosecurity and Public Health Preparedness at the University of Texas School of Public Health.

How and when the flu spreads is dependent on other factors as well: the fitness and efficiency of the virus itself along with its innate ability to replicate; the susceptibility of the host; and the environment, which includes not only the weather, but also human behavior (for example, groups of people confined together inside, making it easier for the virus to jump from person to person).

If a particular virus is especially robust, the weather and other environmental factors may play a lesser role.

"It may not care what the environment is like, because it doesn't need that assistance, or it may depend on environmental factors. It could go either way," Treanor said.

And while influenza virus needs a warm human body to replicate, it seems to sustain itself bette
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