The World Health Organization says it is launching an international investigation of a new strain of bird flu that has claimed 17 lives in China.
Almost 100 other people have been infected -- most near Shanghai.
Until now, the virus was believed to have only been transmitted from birds to humans. But officials say about half of those infected appear to have had no contact with poultry.
The WHO says there is so far no evidence that this latest strain of bird flu -- H7N9 -- is easily spreading among humans.
But Chinese authorities are investigating whether long-term, unprotected exposure to an infected person -- such as a family member -- might result in person-to-person transmission.
The WHO probe is expected to involve investigators from the United States and Europe, in addition to those in China.
China has been working on developing vaccines and other treatment for the virus, as part of a wider plan to combat any potential outbreak. It has also slaughtered thousands of birds and closed many poultry markets in an attempt to slow the spread of the disease.
This is believed to be the first time humans have contracted the H7N9 bird flu virus. It previously existed only in birds. The more common strain of avian flu -- H5N1 -- has killed more than 360 people worldwide in the last decade.
China is considered one of the countries at greater risk for bird flu because it has the world's biggest poultry population and many chickens in rural areas are kept close to humans.