Bird flu virus could evolve to pass from human to human But a recent study of the virus, published in this week's Science journal, shows that with just five specific mutations it can be passed between ferrets, suggesting that with the same variations it could also be airborne among humans. Prof Derek Smith, one of the Cambridge researchers, said: "With the information we have, it is impossible to say what the exact risk is of the virus becoming airborne transmissible among humans. The original study, along with another similar paper in the Nature journal, was held back from publication for several months because American authorities feared the information could pose a security threat. But the paper by Ron Fouchier of the Erasmus Medical Center in Rotterdam highlights that the virus was not deadly to ferrets which caught it by breathing it in, easing initial concerns that it could be made into a bioweapon by terrorists. Prof Wendy Barclay, Chair in Influenza Virology at Imperial College London, said: "How much of a fitness cost these changes confer individually or when combined and in which species, will be important to establish before we can really assess how close we are to the H5 pandemic. Hopefully more work from the biologists to help the mathematicians address how mutant influenza viruses with new properties evolve within a host will increase our ability to estimate the risks of pandemics.