Public health experts around the world are sounding the alarm as cases of a virulent strain of avian influenza called H5N1 rise in mammals. Bird flu has infected humans in the past, mostly people who work directly with diseased poultry, but there has never been widespread human-to-human transmission of the virus. If there were, it could be a catastrophe: The original H5N1 mutation had a 50 to 60 percent mortality rate in humans.  

The latest outbreak of H5N1, which began in the U.S. in late 2021, has resulted in the culling of 58 million birds thus far and led to a marked increase in the cost of eggs and poultry at the supermarket. It's America's second major surge of H5N1since the strain was first detected in southern China in the late 1990s — the first significant U.S. wave kicked off in 2014 and was contained mainly to the Midwest.

Since 2021, H5N1 has been found in at least 47 states. It's circulating among wild birds, cropping up in wild mammals, and, crucially, bouncing between mink. That last development is what really has experts alarmed. More broadly, the H5N1 outbreak fits a pattern scientists have been ringing alarm bells about for years now: Climate change is throwing ecosystems out of whack and spurring the spread of disease, putting wildlife and human health at risk. 

Avian flu viruses are adapted to bind to birds' receptor cells. Humans and other mammals have some avian-like receptors, but they're typically buried deep in the lungs. Because of this anatomical quirk, it would take an enormous load of H5N1 for one infected mammal to dredge up enough of the virus to infect another mammal. Unless, of course, the virus evolved to bind to mammalian cells in the upper respiratory tract. 

That's what appears to be happening now. Late last year, 50,000 mink on a mink farm in Spain were killed when lab tests showed the animals had contracted H5N1. A study published last month said that the virus had been spreading between the mammals, whose respiratory tracts have physiological similarities to humans'. It's the first time such an outbreak has been documented.