It's emerged that virulent H5N1 bird flu was sent out by accident from an Austrian lab last year and given to ferrets in the Czech Republic before anyone realised. As well as the risk of it escaping into the wild, the H5N1 got mixed with a human strain, which might have spawned a hybrid that could unleash a pandemic.
Accidental release of a mixture of live H5N1 and H3N2 viruses could have resulted in dire consequences. While H5N1 doesn't easily infect people, H3N2 viruses do. If someone exposed to a mixture of the two had been simultaneously infected with both strains, he or she could have served as an incubator for a hybrid virus able to transmit easily to and among people.