Wild water birds — including ducks, gulls and shorebirds — are the natural reservoirs for influenza A viruses, which come in a variety of subtypes. Generally, wild birds carry relatively benign versions of these viruses, which pose little immediate threat to birds or people. But flu viruses can change quickly, accumulating new mutations and swapping genetic material. These changes can, and sometimes do, turn a ho-hum virus into a lethal one, like the version of H5N1 that is currently circulating.

Much of the time, flu circulates in shorebirds and gulls at low levels, often turning up in fewer than one percent of samples. But at the Delaware Bay in May and early June, it explodes, passing easily from bird to bird. Over the years, the St. Jude team has found it in 12 percent of their samples, on average, though that figure has climbed as high as 33 percent. They have found almost every subtype of influenza A, in addition to novel remixes, which can emerge when an animal is infected by more than one version of the virus at once.