It’s been almost a month since a case of avian influenza was detected in poultry in the central United States. So it might seem that the epidemic—which over several months caused the destruction of 49.5 million chickens and turkeys—can safely be considered over. But in fact, it may have only taken a break. And if it returns, as some experts predict it will, what one government official calls “the largest animal-health emergency in this country’s history” may turn out to be just an opening act.

At risk, the next time, will be not just the egg and turkey farms of the Midwest, but the billions of birds being raised in the poultry-producing centers on the east and west coasts—effectively, most of the poultry economy of the United States. And attempts to prevent it, by developing a vaccine, may paradoxically turn out to be almost as devastating—because international trading partners say they will bar imports of any birds vaccinated against the disease. The dire uncertainty that surrounds bird flu emerged last week at a hearing held by the Senate Committee on Agriculture to examine how the epidemic unfolded and why it spread. MORE