The World Health Organization has warned that the virus that causes the illness -- if given sufficient opportunity -- would mutate into a form that is highly infectious and easily transmissible from person to person. Such a change could start a global outbreak.

    However, this case -- although it involved the disease apparently passing from one person to another -- does not exactly fit the profile of an infectious human-to-human outbreak, and it has remained something of a puzzle.

    "It has no biological features for human-to-human transmission," said Mao Qun'an, Health Ministry spokesman. An epidemiological investigation showed the father was infected through close contact with his son, he said.

    The cases took place in the provincial capital, Nanjing. The son, 24, and the first to be infected, died on Dec. 2. The father was later confirmed to be infected with the H5N1 virus, which causes bird flu.

    At the time, the ministry said experts had found that the virus that infected the son had originated with poultry and had not mutated. But it remained unclear how the son was infected in the first place, as neither man had any known contact with dead poultry -- the primary known source of the ailment for humans.

    The young man, surnamed Lu, developed fever, chills and other symptoms on Nov. 24 and was hospitalized on Nov. 27 after being diagnosed with lower left lobe pneumonia. His father developed a fever and was hospitalized for lower lobe pneumonia on Dec. 3, the day after his son's death.

    "The father has recovered," Mao said, adding that the cases have been effectively contained.

    Local authorities had kept 83 people who had close contact with either man under close observation but none had shown unusual symptoms so far, according to the ministry.

    The case of the Lu family, although unusual, is not the only one of its kind. Reuters reported last month that a similar case occurred in Pakistan.

    The latest cases bring the number of confirmed human infections of bird flu in China to 27 since 2003, with 17 deaths.

    A human-use bird flu vaccine has been in the second phase of clinical tests in Beijing by the Beijing-based vaccine producer Sinovac Biotech and the Chinese Center for Disease Control and Prevention. 

    It has proved "safe" and "effective" in the test, said Sinovac Biotech in late last month.

    The major index of the vaccine all reached international standard and performed well in human body. None of the test takers were found with serious negative reaction, which proved that the vaccine was safe, it said.

    Bird flu, or Avian influenza, is a contagious disease of animal origin caused by viruses that normally infect only birds and, less commonly, pigs.

    China's Ministry of Agriculture said in early December that the possibilities of regional bird flu outbreaks were "very high" in the winter and coming spring.

    Xinjiang in northwest China has reported an outbreak of bird flu since late December, leading to the death of more than 35,000 poultry.

    The local government said the situation has been under control and no human infection has been found yet.