Last year, when an H5N1 avian flu virus — commonly known as bird flu — was spilling over from bird populations into a variety of wild mammals, Seema Lakdawala, a virologist and influenza A transmission specialist at Emory University, was “not overly concerned” about human risk. We don’t have “much of an interface with seals or with foxes, for that matter, or polar bears,” she says.

But when it comes to cows, that interface is vast. People on dairy farms regularly interact with cows and their milk; when the animals and their milk are infected with a virus that can cause disease in humans, and that mutates constantly, each of those interactions functions as an opportunity for the virus to workshop its adaptability. Now, says Lakdawala, “I am more concerned than I have been, and it’s not for the general public — it is for dairy workers.”

The H5N1 outbreak among cows on 34 dairy farms in nine states has so far led to only one very mild human infection. However, the virus was likely spreading among cows for months before it was detected. Lakdawala’s greatest concern is that this highly changeable virus has now arrived at an important point of human-animal convergence, and that we are not prepared.

For a virus to cause a human pandemic, it has to have three important characteristics, say flu experts. It has to cause human disease; it has to be something our immune systems haven’t encountered before; and it must spread easily among humans, especially through the air. The latest events do not yet demonstrate that H5N1 has new capacities in any of these categories. However, they hint that the virus has the machinery to evolve those capacities — and that it could do so before we know it.

In dairy cows, H5N1 has found an excellent laboratory for evolving traits dangerous to humans

Although Lakdawala was concerned when mink, seals, and other mammals were infected with H5N1 last year and the year before, cows are different. An outbreak among “mammals with a large interface with humans” is a red flag to her.

It’s a numbers game. Although all viruses mutate routinely, flu viruses are particularly good at shapeshifting and can even swap entire chunks of genetic material with other flu viruses if an animal is co-infected with more than one of them. These mutations happen randomly, and most don’t make the viruses more dangerous to humans — but it’s entirely realistic to imagine that some occasionally do. If that occasionally-human-threatening mutation happens to a flu virus that’s infected, say, a wild fox, it doesn’t........

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