For an avian flu and a human flu to mix it up, they have to infect the same animal. Scientists have long considered the pig a likely mixing vessel, because pig cells have surface molecules that allow entry to both kinds of virus. A pig could conceivably catch a human flu from a farmer and a bird virus from, say, ducks at the same farm. The two viruses could then "reassort," creating a hybrid that—in the worst case—would now be able to infect human cells while still carrying bird-virus genes that would make it radically new to the immune system of the people who catch it, and unusually virulent.

Reassortment explains the two lesser flu pandemics of the 20th century, in 1957 and 1968. In each year a new flu subtype appeared, combining genes from the human virus that had been causing mild outbreaks in prior years with new genes from a bird virus. The new pandemic viruses raced around the world, together killing about two million people.

But in 1918, Taubenberger now believes, something different happened. "We think it's pretty likely that the virus was not derived from a previously circulating human virus," he says. All of its genes mark it as an animal virus, pure and simple, that somehow crossed to people without the help of genes from a previous human strain.

Now H5N1 is doing the same thing. So far, its steps across the species barrier are tentative, which is why it has caused tens of deaths, not millions. But as in 1918, doctors who have seen its effects close up are shaken.

The x-rays tell the story as Tran Tinh Hien, a doctor at the Hospital for Tropical Diseases in Ho Chi Minh City, clips them to a light box. In the first image, made the day the 18-year-old girl was admitted with bird flu, a whitish cloud appears at the base of her rib cage. Her lungs were partly filled with fluid. In a second film, four days later, the haze has spread throughout her chest. "All the lung tissue was destroyed," Hien says. "The process still happened when we treated." A week later the girl was dead.

So it went for Hien and his staff throughout January, when the latest bird-flu outbreak reached its peak in southern Vietnam. They cleared a 50-bed ward normally reserved for malaria and dengue fever and turned it into an isolation unit.

They sustained patients with oxygen masks and ventilators and treated them with oseltamivir, or Tamiflu, an expensive antiviral drug that can fight H5N1. Nurses worked 24-hour shifts, gowned, masked, and goggled for protection against the virus. As Hien says: "We were pushed to the wall."

He and his staff did everything they could for their nine bird-flu patients. "Unfortunately," he says, "we could not save any lives."