Since most new strains of flu originate in China, the boy's doctors were accustomed to sending throat cultures to major infectious disease laboratories. Working through the weekend, the Centers for Disease Control and Prevention (CDC) in Atlanta returned disturbing news: The disease had been caused by a strain of bird flu. It was the first case of human influenza attributed to a purely avian flu (although the strain had already killed millions of chickens).

Virologists have learned to watch for corpses when a virus "jumps" from one species to another. That's because viruses often coexist with "host" species (like the unknown bird whose virus infected the Hong Kong boy). But they can be deadly if they infect another species -- the unlucky boy -- which lacks the right antibodies. (The Why Files covered inter-species infections.)

Global flu epidemics (called pandemics) have started with another form of species jumping, when a virus containing genes from a non-human species -- say a pig or a bird -- gets loose in the human population.

Is flu on the warpath once again?
Probably not yet, says Nancy Arden, a CDC epidemiologist who's followed the Hong Kong incident. Scientists have not found the bird virus in more than 4,000 throat samples taken from Chinese respiratory-disease patients since the outbreak. "It appears not to have been spreading among humans," she says. But she notes that "this is the first time that a purely avian strain of influenza has been found to cause the flu in humans, and that's why there was such an intensive investigation."

The reassuring throat cultures were all taken from patients who had flu symptoms. Now investigators are looking for anyone who might have recovered after getting the bird flu.

Viruses can't cause pandemics unless they have two qualities, Arden notes. First, they must be able to infect humans, as this virus apparently has. Second, they must be able to spread easily among the human population. For unknown reasons, the bird virus lacked this vital talent.

So the "epidemic" had just one victim?
Yes. And it's also true that the boy didn't die of flu alone. He also had Reye syndrome, a sometimes-fatal ailment caused when young people with fevers take aspirin.

The global strategy to quickly identify new influenza strains worked this time, and the effort to rapidly search for more cases was promptly started. But despite the low death toll, the case reminds us of the virility of viruses, and points up the folly of relaxing our guard against infectious disease.

The flu scare is not the only virus that's caught our attention recently. Earlier this year, there was a deadly outbreak of Ebola virus in Africa. Dengue fever continues moving northward in the Americas. And the slow-acting AIDS virus (here's our coverage) is not exactly under control, either.

(The bibliography has more on the 1918 epidemic in "The Dead Zone," and on the death in Hong Kong in "The Flu Pandemic.")