Einstein's at work
Countries around the world were destroying vials of a nearly 50-year-old killer flu virus Wednesday that were sent to thousands of labs as part of a routine test kit, raising fears of a global pandemic.
Unlike the avian flu, which doesn't effectively infect humans (yet), this is the exact strain that caused a worldwide pandemic in 1957. So if it got out, it could spread rapidly between humans, an ability the avian flu has yet to acquire. Of course, this report notes that 70% of poultry in Vietnam are infected with the H5N1 virus, suggesting it is just a matter of time before it jumps to the human population
This will most likely occur when someone is simultaneously infected with the avian variant and more mundane flu. The viral genes can then recombine to form a hybrid strain. This strain will infect humans efficiently, but not be recognized by the immune system because it doesn't resemble previous influenze strains that infected humans.