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 one-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 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 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.
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, 20pc of victims have died, 20pc are recovering and the rest remain ill. In fatal cases, the virus has triggered sepsis 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."
Scientists have learned lessons from the H5N1 bird-flu strain which also emerged in China, and since 2003 has led to 628 confirmed cases in 15 countries and 374 deaths.
With a 60pc mortality rate, H5N1 is even deadlier than the new strain. Luckily it has not yet shown any sign of making the jump to human-to-human transmission.
Although there is no reason to think that H7N9 will acquire this ability any time soon, it already displays some degree of human adaptation.
In H5N1, five key genetic changes were identified that scientists believe are needed for the virus to become a potential pandemic threat.
Laboratory studies have revealed two of these mutations in H7N9. One of them is integral to the "H7" part of its structure and enables the virus to latch onto certain receptor molecules in the human respiratory tract. It is seen even when the virus is confined to chickens.
The other has only been acquired after human infection and helps the virus to replicate inside cells.
How easy or difficult it might be for the other three changes to occur, and to what extent H7N9 needs the same mutations as H5N1 to pass between humans, is unknown.