Pigs are especially important to infecting humans with avian influenza viruses. The cells lining their lungs have receptors that match up with both bird and human viruses, so those lungs turn out to be perfect places for influenza strains to “meet each other” and mix. It is even possible to have a triple reassortment, where strains of all three species—humans, birds, and pigs—mix to form a completely unpredictable new influenza virus. When that happens, it’s a spin of the genetic roulette wheel whether the new strain is more or less serious than the strains from which it emerged. In 1918, that
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