The Spanish influenza virus pandemic of 1918 killed more than 20 million people worldwide. In September 7 Science, Mark Gibbs and colleagues from the Australian National University in Canberra propose that the pandemic was the result of a recombination between swine-lineage and human-lineage viral strains (Science 2001, 293:1842-1845).

They analysed sequences of the hemagglutinin (HA) gene from 30 H1-subtype influenza isolates, using the sister-scanning method and a maximum likelihood method. They suggest that recombination replaced a central region of the human-lineage HA gene with sequences from the swine-lineage virus.

Gibbs et al. propose that the recombination event occurred just before outbreak of the pandemic and that HA recombination affected antigenicity and viral virulence.

In the same issue of Science, Masato Hatta and colleagues from the University of Wisconsin-Madison describe the molecular basis for virulence of the H5N1 influenza strain that caused the 'flu outbreak in...

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