Four times in the past century, a new strain of flu has emerged that can spread quickly in humans. One of those strains, which emerged in 1918, killed an estimated 50 million people.

All human flu strains evolved from flu viruses that live in birds. To understand how these transitions happen, scientists have recently been tinkering with a strain of bird flu to see how many mutations it takes until its spreads from mammal to mammal.

When news of their efforts emerged last fall, a fierce debate broke out about the wisdom of publishing the experiments in full.

Eventually, the scientists got the go-ahead from a federal advisory board, and earlier this year they described how a few mutations of a strain called H5N1 enabled it to spread among ferrets. But the controversy still rages: Responding to worries about an accidental release of an engineered virus, leading flu scientists agreed in January to a moratorium on further research, and experts are debating when it should be lifted.