What is H5N1?

The virus that’s spreading now is a strain that was first found in the Netherlands in 2020. As of December, the World Health Organization had collected reports of six human infections with this particular virus: the Colorado case, three others that were asymptomatic or mild, one that was severe and one that was fatal.

Genetic sequencing from those six cases didn’t show any signs the virus had picked up mutations that would be useful in spreading between humans.

Other species that have gotten infected around the world include a badger, bobcat, coyote, dolphin, ferret, fisher cat, fox, lynx, opossum, otter, pig, polecat, porpoise, raccoon, raccoon dog and multiple minks and seals.

Before 1997, when there was an outbreak of a different H5N1 virus in Hong Kong, it was believed bird flu couldn’t jump to humans, said Gary Butcher, a professor at University of Florida who studies poultry viruses.

Since then, several other avian influenza viruses have sporadically affected people, but none has come close to starting a pandemic. It’s likely the same thing has happened throughout human history, but wasn’t recognized until we developed genetic sequencing, he said.

“The bird flu wasn’t anything new,” he said.

Since 1997, 880 people were confirmed to be infected with that form of H5N1, and about half died. That’s a small number, but an incredibly high rate — though it’s possible that people who weren’t as seriously ill never sought testing and didn’t find out which flu strain they had. Most of those infected worked with poultry, though there was some limited transmission within families that shared genetic risk factors, Butcher said.

The seasonal flu has killed an estimated 18,000 people in the United States since October, according to the Centers for Disease Control and Prevention. Most years, it kills 30,000 or more.