Virology. A molecular whodunit.

The Spanish flu pandemic killed more than 20 million people worldwide in 1918. An extremely virulent influenza virus that jumped from chickens to humans appeared in Hong Kong in 1997. Why is it that these particular flu viruses were so virulent? In an enlightening Perspective, Webster discusses new work ( Hatta et al., Gibbs et al.) that offers fresh insights into the factors determining flu virus virulence. In addition, he advises on the steps that we need to take to avert another flu pandemic on the scale of the Spanish flu.