In the first two months of 2006, the spread of avian flu strain H5N1 in Africa and Europe fanned fears that it might also spread among humans. Yet despite killing more than half the humans it infected, H5N1 has been implicated in little more than 150 deaths since 2004. This year virologists began

In March University of Wisconsin virologist Yoshihiro Kawaoka looked for H5N1 receptors in the human respiratory tract and found them only deep within the lungs, on the tiny air sacs through which oxygen passes into blood. That deep location would make it difficult for an infected person to spread the avian flu virus through coughing or sneezing.

When it does infect, though, H5N1 is a killer. A reconstruction of the 1918 flu, which killed more than 20 million people, may explain why. When University of Washington School of Medicine virologists John Kash and Michael Katze infected mice with a reconstructed 1918 virus, the animals produced high levels of cytokines, chemical messengers that trigger a powerful immune response. In a separate study, Menno de Jong of the Oxford University Clinical Research Unit in Ho Chi Minh City, Vietnam, found higher levels of cytokines in tissues from H5N1 victims than in those with ordinary seasonal flu.

The implication is that avian flu, like the 1918 flu virus, may trigger an unusually intense, potentially lethal inflammatory response. Such a reaction is likely to be stronger in young, healthy adults, Kash notes, which could help explain why H5N1, like the 1918 flu, is just as adept at killing young adults as it is the elderly and very young.