This year, flu is again in the news. A lethal strain of avian influenza—bird flu—is spreading. Because this strain can infect humans as well as birds, scientists fear that in a worst-case scenario the avian flu could trigger a human pandemic.

In this context, influenza is a challenge to public health officials. But to scientists, the virus also presents a unique opportunity—a way to study evolution in action.

A worker processes a chicken at a processing plant in Shanghai. In November, China confirmed its first cases of avian flu in humans and, to try to stem the spread of the virus, rushed to vaccinate billions of chickens and other fowl.

Photo: Stuart Ramson/AP