H1N1 is likely lurking in some animal reservoir right now. "It's probably in an animal host that is not pathogenic to the animal," said Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City.
In recent weeks, outbreaks of a different strain of influenza -- H3N2 -- have been spotted in Iowa.
And it's possible that will be a dominant strain this year. The Southern Hemisphere, which is wintering right now, is seeing a mixture of H1N1 and H3N2, Walsh said.
"We might see very little H1N1 and more H3N2 or Influenza B, although I think the chances of that should be significantly less than [a season dominated by H1N1]," he noted.
With H1N1, there's a good possibility that much of the U.S. population will be immune, either because they were sick last year or because they received the vaccine, Walsh added.
The flu is notorious for mutating each year, however.
"Very often, let's say if two . . . flu viruses are in the same host and they do a reassortment of genetic material where they sort of exchange genetic material and therefore genetic character, it's possible that H1N1 will take on more virulence," Horovitz said.
Still, "we can't predict if H1N1 will evolve in such a way that it circumvents the immunity that has developed and also cause a big outbreak," Walsh cautioned.
The U.S. Centers for Disease Control and Prevention has more on H1N1.
SOURCES: Len Horovitz, M.D., pulmonary specialist, Lenox Hill Hospital, New York City; Edward Walsh, M.D., professor, infectious diseases, University of Rochester Medical Center, Rochester, N.Y.; Scott Lillibridge, M.D., assistant dean, Texas A&M Health Science Center School of Rural Public Health, Houston, and executive director, National Center for Emergency Medical Preparedness and Response
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