A deadly bird flu virus sweeping through China has taken the first steps towards becoming a global threat to human populations, experts have revealed.
In the space of one month, the avian strain known as H7N9 has spread through all 31 Chinese provinces and claimed 125 victims, killing a fifth of those infected.
Scientists say it is mutating rapidly and already has two of five genetic changes believed to be necessary for human-to-human transmission.
Currently the virus has made its home in chickens, and only affected people who have had close contact with the birds, often at live markets. If it were to become fully adapted to human hosts, it could result in a serious worldwide pandemic claiming millions of lives.
The 1918 Spanish Flu pandemic - the worst flu pandemic in modern history - killed up to 80 million people worldwide and is believed to have originated in birds.
Experts speaking in London said there was no room for complacency over H7N9, and warned against the folly of assuming it was a far-away foreign problem. GPs have been sent letters advising them on how to identify cases and what action to take if one is suspected.
Research is ongoing into how the virus is behaving and what makes it so virulent. The virus has infected people of all age groups, from two to 81, suggesting that humans have no natural immunity to it.
So far 20% of victims have died, 20% are recovering and the rest remain ill. In fatal cases, the virus has triggered sepsis - a massive inflammatory response - leading to multiple organ failure.
Leading British expert Professor Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London, said: "This is a very, very serious disease in those who have been infected. So if this were to become more widespread it would be an extraordinarily devastating outbreak.
"It's very unusual to see more than 100 new cases in a very short time period. I think it's definitely something we need to be concerned about."