In nature, the flu virus is found in wild aquatic birds, such as ducks and shore birds. The flu has persisted in these birds for millions of years and does not typically harm them. However, the frequently mutating flu viruses can readily jump the species barrier from wild birds to domesticated ducks and then to chickens. From there, the next stop in the infectious chain is often pigs.
Pigs can be infected by both bird (avian) influenza and the form of influenza that infects humans. In a setting such as a farm where chickens, humans, and pigs live in close proximity, pigs act as an influenza virus-mixing bowl. If a pig is infected with avian and human flu simultaneously, the two types of virus may exchange genes. This flu virus can sometimes spread from pigs to people.
Depending on the precise assortment of bird-type flu proteins that make it into the human population, the flu may be more or less severe. For the first time in 1997, scientists found that bird influenza skipped the pig step and infected humans directly. Alarmed health officials feared a worldwide epidemic (a pandemic), but, fortunately, the virus could not pass between people and thus did not spark an epidemic.
(Click Avian Flu for more information about this type of flu.)
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