Chinese authorities said Wednesday that a 73-year-old woman had died after being infected with a bird flu strain, the first time that particular strain has made a human sick.

The Chinese Center for Disease Control and Prevention confirmed that the woman, in the city of Nanchang, had been infected by the H10N8 virus, a strain that had not previously been found in people, the Jiangxi province health department said on its website.

The World Health Organization called the development worrisome.

This is the second new bird flu strain to emerge in humans this year in China. In late March, the H7N9 virus broke out, infecting 140 people and killing 45, most of them on the mainland.

The outbreak was controlled after the country closed many of its live animal markets. Scientists had assumed the virus was infecting people through exposure to live birds.

Timothy O'Leary, spokesman for the World Health Organization's regional office in Manila, said WHO officials were working closely with Chinese authorities to better understand the new virus. He said that though the source of the strain remains unknown, birds are known to carry it.

He added that it would not be surprising if another human case was detected.

"It's worrisome anytime a disease jumps the species barrier from animals to humans," O'Leary told The Associated Press by phone. "That said, the case is under investigation (by Chinese authorities) and there's no evidence of human-to-human transmission yet."

The potential for bird flu viruses to mutate and spread from person to person is what worries health experts. Such an event would likely cause many people to fall ill. Right now, contact with an infected bird is how humans get the virus.

In the new case, the Jiangxi health department said the woman had severe pneumonia before dying Dec. 6 in a hospital in Nanchang.

She had suffered from high blood pressure, heart disease and other underlying health problems that lowered her immunity, the health department said. Her medical history showed she had been in contact with live poultry.

The health department said "no abnormalities" have been found in people who had close contact with her. It did not say if they had been tested or quarantined, though China has taken those measures in previous outbreaks.

Experts are cautious when it comes to bird flu viruses infecting humans. They have been closely watching the H5N1 virus, which has killed 384 people worldwide since 2003.

Early this month, at least two cases of H7N9 cropped up in Hong Kong for the first time. The disease had previously been contained to mainland China and Taiwan.

Al Jazeera and The Associated Press