X-rays showed the shadow on the patient's lungs had diminished, but he was still critically ill, said the Health Bureau of Shenzhen City.

The 31-year-old patient surnamed Jiang was confirmed by the Ministry of Health to have contracted bird flu on June 15, bringing China's total human infections to 19.

Jiang had been undergoing treatment for eight days in a local hospital, which had the most advanced intensive care unit in the city, said the bureau.

Meanwhile, medical observation of 98 people who had close contact with him had found no suspected symptoms, such as pneumonia or bird flu-like symptoms, said a statement from the Shenzhen Center for Disease Control and Prevention.

Bird flu has killed 12 Chinese since last year. The disease has also infected 228 people and killed 130 worldwide, according to figures from the World Health Organization (WHO).

Experts fear the H5N1 strain of the virus could mutate to become transmissible among people, causing a global pandemic. To date, most human cases had direct or indirect contact with infected birds.

Indonesian Health Minister Siti Sufari Fadillah announced Wednesday that the investigation conducted by a WHO team on the world's largest cluster death of bird flu cases in the country's North Sumatra province found the virus was transmitted from animal to human, not from human to human.

Seven people in the same extended family died from the highly pathogenic H5N1 virus last month, followed by the deaths of two siblings in Jakarta, which the minister described as a cluster.

It was the biggest reported cluster of deaths, raising international concern on possible human transmission in Indonesia.

Senior Chinese health expert Shu Yuelong said Wednesday there was no evidence of human-to-human transmission in China, but warned the evolution of the virus was unpredictable.

No trace of human influenza had been found in the gene of the virus extracted from Chinese bird flu patients either, said Shu, director of the National Influenza Center under the Chinese Center for Disease Control and Prevention.

Hong Kong's health chief, York Chow, said on June 16 that the latest human case of bird flu on the Chinese mainland might indicate the virus had mutated and become as infectious in summer months as it is in cooler months.

But a WHO official said it remained unclear whether there were truly distinctive seasonal patterns to outbreaks of the bird flu in poultry.

"We do know that the bird flu virus can survive for a time in colder weather, but it's really not clear at this point whether the virus is changing in such a way that it can survive in warm weather for a longer period than it was previously able to," Roy Wadia, spokesman for the WHO office in China, told Xinhua.

"This is why it is so very, very important that agricultural authorities anywhere share virus isolates bird flu from animal outbreaks with the international scientific community," he said, urging the Chinese Ministry of Agriculture to share isolates from 2005 and 2006 as soon as possible.