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.

Last December, the Austrian branch of US vaccine company Baxter sent a batch of ordinary human H3N2 flu, altered so it couldn't replicate, to Avir Green Hills Biotechnology, also in Austria. In February, a lab in the Czech Republic working for Avir alerted Baxter that, unexpectedly, ferrets inoculated with the sample had died. It turned out the sample contained live H5N1, which Baxter uses to make vaccine. The two seem to have been mixed in error.

Markus Reinhard of Baxter says no one was infected because the H3N2 was handled at a high level of containment. But Ab Osterhaus of Erasmus University in the Netherlands says: "We need to go to great lengths to make sure this kind of thing doesn't happen."

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.