AP Photo

Washington

THE swine flu outbreak seems to have emerged without warning. Within a few days of being noticed, the flu had already spread to the point where containment was not possible. Yet the virus behind it had to have existed for some time before it was discovered. Couldn’t we have detected it and acted sooner, before it spread so widely? The answer is likely yes — if we had been paying closer attention to the human-animal interactions that enable new viruses to emerge.

While much remains unknown about how pandemics are born, we are familiar with the kinds of microbes — like SARS (severe acute respiratory syndrome), influenza and H.I.V. — that present a risk of widespread disease. We know that they usually emerge from...