Experts have warned for months that the H5N1 strain of bird flu could change into a form that spreads easily between people, a development that would spark a pandemic.
Yet no one can say precisely when or how this will happen, and as Declan Butler reports in this week's Nature, there is not even universal agreement that it will.
Some virologists believe that if H5 viruses could mutate into a pandemic form, they would already have done so, as they have been infecting humans for decades. This, they say, means these viruses are inherently incapable of spreading from person to person.
Others counter that this goes against evolutionary theory and add that new studies suggest H5 is not widespread among humans.
One thing everyone agrees on is that nobody knows for sure what will happen, except that future flu pandemics are inevitable, whether caused by H5N1 or another virus.
Even one of the most vocal opponents of the H5N1 pandemic scenario, Peter Palese of the Mount Sinai School of Medicine in New York, says that preparations for future pandemics have not been given enough attention.