How viruses evolve

Change can be so quick that scientists often only become aware when it's too late:

[V]iruses can evolve entirely new traits — and thus give rise to new diseases. A debate has swirled around whether a strain of avian flu called H5N1 could turn into a global killer. Last year scientists ran experiments in which a highly lethal H5N1 virus gained the ability to spread among mammals. In response to the urging of a federal advisory board, the scientists will withhold crucial details when they publish their research. But according to news reports, the team, based in the Netherlands, found that it took five mutations to transform the flu.