A World Health Organisation flu expert says the new bird flu strain in China is "unusually dangerous''.
"This is definitely one of the most lethal influenza viruses we have seen so far," Dr Keiji Fukuda said at a news conference in Beijing.
"When we look at influenza viruses this is an unusually dangerous virus."
Dr Fukuda said that ased on the evidence, "this virus is more easily transmissible from poultry to humans than H5N1", a strain which spread in 2003.
According to the WHO, the H7N9 flu has infected 108 people in China since it was first detected in March. At least 22 people are dead.