For two years now, public health experts have been nervously monitoring the progress of a virulent strain of influenza known as H5N1, which has killed 140 million birds, mostly in Asia. Humans catch it too100 or so have been hospitalized with the illness worldwide, and about 60 have diedbut since this flu doesn't pass directly from one person to another, it isn't a serious threat so far. If it mutates, though, to allow easy human-to-human transmission, scientists fear it could rival the legendary Spanish flu pandemic of 1918 and 1919, which ravaged the planet, killing more than 50 million people.
A series of papers published this week in the journals Science and Nature suggest that such a mutation is all too plausible. Working from snippets of tissue from victims of that long-ago plagueone from an Inuit woman buried in permanently frozen soil in Alaska, two others from autopsy samples taken from American soldiers who died in the Northeastscientists from the Armed Forces Institute of Pathology, the Centers for Disease Control and other institutions determined that the lethal Spanish flu was itself a bird flu that evolved to infect humans directly.
None of the tissue samples contained an intact virus, but the researchers used the fragments they found to recreate the lethal virus, which they tested in mice and on human lung cells to confirm its virulence. They discovered that the 1918 bug was different in one crucial way from the viruses responsible for the flu pandemics of 1957 and 1968, better known as the Asian and the Hong Kong pandemics. (A pandemic is a worldwide epidemic.) Those were human influenza viruses that had incorporated a few bird flu genes, but the 1918 virus was a bird flu that changed to permit more efficient human-to-human transmission. It wasn't that many changes, either. Dr. Jeffery Taubenberger, one of the investigators, estimates that it took only about 25 mutations to cause that changenot a very large number, considering that influenza viruses mutate relatively easily. (That's why at least one new strain appears nearly every year).
It isn't yet clear precisely which mutations are key to turning a lethal bird flu into a worldwide human health crisis. And it's by no means certain that H5N1 will make that deadly leap. But sooner or later, experts say, it will happen. That's why 65 nations are meeting today and tomorrow in Washington to try and figure out how to respond when it does.