Identifying novel flu strains in animals is one thing; determining whether they pose a human danger is another. “I’m a lot more pessimistic about being able to predict these things,” says Jeffery K. Taubenberger of the National Institute of Allergy and Infectious Diseases. In March he published an analysis of two swine branches of the H1N1 family tree—one of them a Eurasian strain that contributed segments of the novel H1N1 now infecting humans. The two strains had a common H1N1-type ancestor, but both have been evolving independently in pig populations, and the minute changes to viral genes that allowed the virus to adapt to a new host were different in each strain. Many other scientists looking for consistent signals that a virus is changing hosts or is becoming more transmissible or more virulent have also failed to find clear patterns.

As a result, no one can explain why the avian H5N1 flu virus has infected some 400 people worldwide, mainly in Asia and Africa, but failed so far to adapt completely to humans. Nor do scientists know where the original 1918 pandemic virus came from or where its distant descendant, the new H1N1 strain, is going. Having spread to 40 countries, infected nearly 10,000 people and killed 79 as of late May, it might still fizzle in the coming months or learn to transmit between people more easily. And this fall it could return to the Northern Hemisphere as a lion or a lamb.

Taubenberger, who with his colleagues at the Armed Forces Institute of Pathology first fished the 1918 H1N1 pandemic strain out of preserved samples of victims’ tissue in 1996, says too much is still unknown about the basic biology and ecology of flu viruses. He thinks surveillance of an entire rural ecosystem—pigs, birds, people, as well as dogs, cats, horses, and other domesticated and wild animals—would finally yield some deeper insights into why and how flu viruses evolve.

Fortunately, money and research directed toward pandemic preparations have dramatically improved human flu surveillance and response systems. Richt points to how rapidly labs identified the first U.S. cases of the new flu in two children in southern California and alerted the CDC, allowing health officials to swing into action. Unfortunately, without closer monitoring of the animal sources of novel flu strains, human surveillance will have to remain the first line of defense.

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