An Indonesian girl died of bird flu in the country hardest hit by the virus, health officials said Wednesday, while Vietnam confirmed its first human infection in a year and a half.

The disease racing through Asian poultry stocks remains hard for people to catch, but experts fear it could mutate into a form that spreads easily among humans, potentially triggering a pandemic that could kill millions.

Indonesia is seen as a potential hotspot for that to happen because of its high density of human and chickens, coupled with a general lack of public awareness about the H5N1 virus.

The 5-year-old girl who died last Thursday in Solo, a city in Central Java province, had been suffering from bird flu symptoms for more than a week, said Health Ministry official Muhammad Nadhirin.

Authorities believe she was infected by sick chickens, with 20 dropping dead around her home in recent weeks, he said.

Bird flu has killed at least 185 people since late 2003, according to the World Health Organization, the bulk of them in Indonesia, which has tallied 77 deaths, and Vietnam, where 42 have died.

A 30-year-old Vietnamese man from northern Vinh Phuc province remained hospitalized Wednesday, days after testing positive for the virus, said Tran Quy, former director of Bach Mai hospital in Hanoi.

"The man is in critical condition," he said. "He has a high fever, difficulty breathing, coughing and the X-ray of his lung was completely white. All these are typical bird flu symptoms."

Quy said the man was admitted to the hospital last week. Relatives said he had helped prepare chickens for a wedding reception before falling ill.

It was the country's first human H5N1 infection since November 2005 and comes as bird flu outbreaks among poultry have been reported in southern, central and northern Vietnam.

Samples of the virus will be shipped to a World Health Organization lab for analysis and reconfirmation, said WHO country representative Hans Troedsson.

Vietnam had been hailed a success story for beating back the virus that began ravaging Asian poultry stocks nearly four years ago. A nationwide mass poultry vaccination program, coupled with strong political will, brought the virus under control after it killed dozens of people.

No poultry outbreaks were reported in 2006, but the virus flared again early this year.