The rapid emergence of the H1N1 strain of influenza in North America and its subsequent global spread have reminded the world that viruses and other microbes are often not limited to specific species and have little regard for international boundaries. While the world was watching to see if the highly pathogenic H5N1 influenza, the so-called bird flu, in Asia and Africa would mutate to make transmission easy among humans, the new H1N1 influenza -- first called swine flu because it shares several gene sequences with influenza viruses found in pigs -- seems to have already made that transition.
Changes to influenza viruses can happen as a result of quick mutations or as the end result of a slower drift in genetic material. At the same time, different influenza viruses exchange entire genetic segments with one another when they simultaneously infect the same person or animal. One of the virus's genetic segments controls the hemagglutinin antigen, which sits on the surface of the virus and allows it to attach itself to the cells of the host. Another segment controls neuraminidase, a protein that enables the virus's release from the host cell after multiplying. The combination of H's and N's provides the name used for strains of influenza viruses such as H5N1 and H1N2.
The host, whether it is a person, bird, or pig, provides a place for influenzas to make contact, exchange genetic material, and form new viruses. Such genetic intermixing is not uncommon: the new H1N1 influenza virus, for example, appears to have formed from components of previously circulating viruses with a mix of influenza genes for three different host groups: birds, pigs, and humans.
This specific H1N1 virus rapidly circulating around the world, then, is not a new phenomenon but simply a new strain of influenza. This does not, however, mean that there is no need for concern. Each new strain of influenza has the potential to cause mild to very serious disease,