In 2004 the World Conservation Union placed three vultures—the long-billed, the slender-billed and the Oriental white-backed—on the critically endangered list. Populations of all three reached nearly 40 million in India and South Asia in the early 1990s but had fallen by more than 97 percent. The reasons for saving these vultures from extinction could be framed in familiar terms: we have an ethical obligation to save the world’s biodiversity for its own sake. But the reasons could also be outlined in a less familiar way.

For a long time, observers did not know what was causing the vultures’ decline. Some speculated the culprit was habitat loss or pollution. Several years ago researchers discovered that the birds were being killed by an anti-inflammatory drug, diclofenac, commonly administered to cows. In bovines and humans, the medicine reduces pain; in vultures, it causes renal failure. As the vultures have disappeared, hundreds of thousands of cow carcasses customarily left for the birds have festered in the sun, where they incubate anthrax, according to some reports, and are consumed by dogs. Because of the ready food supply, the feral dog population has exploded—and with it the threat of rabies. Thus, the vultures’ fate may be linked with that of millions of people; saving the vultures from extinction would protect people from dangerous disease.