The rapid emergence of a new strain of influenza (Swine flu) in Mexico, the U.S., Canada, and other countries is quickly reminding the world that viruses and other microbes are not limited by differences in species or political boundaries.

While we have been watching for several years to see if the H5N1 strain of avian influenza (“bird flu”) in Asia and Africa changes to transmit easily among humans, a new variant in the western hemisphere seems to have already made that transition. This new strain, Swine Flu, appears to have been formed from components of four viruses from three different animal species: a bird virus, two pig viruses, and also a human influenza virus.
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