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How worried should we be about bird flu? It’s a question that I’ve been asked by friends and colleagues several times over the last couple of weeks. Their concerns have been spurred by some potentially worrisome developments in the US, including the continued spread of the virus among dairy cattle, the detection of the virus in a pig as well as cow’s milk, and—most concerning of all—the growing number of human infections.

Virologists have been especially worried about the virus making its way into pigs, because these animals are notorious viral incubators. “They can become infected with swine strains, bird strains and human strains,” says Brinkley Bellotti, an infectious disease epidemiologist at Wake Forest University in North Carolina. These strains can swap genes and give rise to new, potentially more infectious or harmful strains.

Thankfully, we haven’t seen any other cases in pig farms, and there’s no evidence that the virus can spread between pigs. And while it has been spreading pretty rapidly between cattle, the virus doesn’t seem to have evolved much, says Seema Lakdawala, a virologist at the Emory University School of Medicine in Atlanta, Georgia. That suggests that the virus made the leap into cattle, probably from birds, only once. And it has been spreading through herds since.

Unfortunately, we still don’t really know how it is spreading. There is some evidence to suggest the virus can be spread from cow to cow through shared milking equipment. But it is unclear how the virus is spreading between farms. “It’s hard to form an effective control strategy when you don’t know exactly how it’s spreading,” says Bellotti.

But it is in cows. And it’s in their milk. When scientists analyzed 297 sample...