Investigators were unsure how the son contracted the disease, Xinhua, the official government news agency, reported. Neither man was known to have had contact with poultry.

The son, 24, died in early December. The father, 52, who became ill just after his son's death, has recovered. Both men lived in Nanjing, the provincial capital of Jiangsu.

Health officials said they were puzzled by the case.

"It has no biological features for human-to-human transmission," said Mao Qun'an, a spokesman for the Health Ministry.

The H5N1 strain of avian influenza has killed millions of birds around the world since it was first identified a few years ago. While the World Health Organization warns that the virus could mutate to allow human-to-human spread, most human cases so far have involved contact with infected poultry.

Health officials have tracked more than 80 people who had contact with the son and father and found no further cases of infection.