Bird flu, or avian influenza, is a group of flu viruses that primarily infect birds. The particular virus in these recent cases, H5N1, was first identified in 1996 in geese in China. The first report of human infection happened in Hong Kong in 1997, where it infected 18 people, killing 6.
There’s been no stopping H5N1, which has continued to evolve and is highly pathogenic—meaning it has the ability to cause severe disease and death. Avian flu viruses tend to be picky about their hosts, typically sticking to one kind of wild bird. But this one has rapidly infiltrated a wide array of birds and animals, from squirrels and bottlenose dolphins to polar bears and skunks.
The blow to sea mammals—and to the dairy and poultry industries—is worrying enough. But a bigger concern, experts say, is what these developments could indicate: The virus is adapting to mammals, edging closer to being able to spread widely from person to person.
“Every time [the virus] jumps to a mammal, there’s an opportunity for tiny changes” that may help it more easily infect people, says Meghan Davis, an associate professor at the Johns Hopkins Bloomberg School of Public Health.
Past pandemics—including, many experts say, Covid-19—have often begun with a virus “spillover” from animals to humans (see “From Animals to Us,” below).