The Rampaging Avian Influenza Is Entering Unknown Territory Highly lethal and spreading widely, the avian flu outbreak has scientists wondering what it will do next.The type of influenza virus that infects birds—known as influenza A, or bird flu—is traditionally mild; most birds don’t show any sign of sickness.Hill wishes influenza scientists had come up with a simple nomenclature, like the Greek alphabet used to name COVID-19 strains, “but we just didn’t,” she says.Working backward, the next part of the virus’s name, HPAI, stands for highly pathogenic avian influenza, a form of bird flu that kills more than 75 percent of chickens in a laboratory setting in less than 10 days.That kind of lethality, Hill says, “never, ever, ever, ever [evolves] in wild birds.” H5N1-HPAI-clade 2.3.4.4b started out as a milder influenza, a so-called low pathogenic avian influenza circulating in the wild."