A devastating bird flu pandemic seems close to be a real threat, as scientists claim to have found the deadly virus is just "three mutations" away from evolving into a strain that could easily transmit from human to human.
Avian H5N1 influenza can currently only be transmitted to humans from birds, meaning it cannot spread quickly through the air between large groups of people.
But, researchers at the Cambridge University found that there are already some strains which are just "three mutations" away from being passable form one human to another.
It suggests the airborne strain could evolve naturally in the wild – even in one person, the researchers said.
Prof Derek Smith, one of the study authors, said the research has shown that the risk – like an earthquake fault line – did exist but it's difficult to say if and when it might become reality.
"With the information we have, it is impossible to say what the exact risk is of the virus becoming airborne transmissible among humans," he was quoted as saying by the Daily Telegraph.
"However, the results suggest that the remaining three mutations could evolve in a single human host, making a virus evolving in nature a potentially serious threat."
Prof Smith said that the research meant we knew what was coming which helped "immensely".
The study was held back from publication for several months as American authorities feared the information could pose a security threat.
But the paper 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," he said.