By sequencing its genes, scientists have traced the virus’s origins to four separate strains. The oldest of these strains has been infecting pigs since the 1918 pandemic (it’s possible the pigs got it from us). A second strain emerged in the 1970s, when a bird flu infected pigs in either Europe or Asia. And a third strain jumped later from birds to pigs in the United States. By the late 1990s, all three strains had combined into a new form, called a “triple reassortant” by scientists. Unnoticed at the time, it silently moved from pig to pig in large closed barns. The triple reassortant then
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