Health authorities in China say the search for a causal link between a son and his father, both victims of bird flu, has revealed no evidence that the virus has mutated into a new strain.
The 52-year old father, a native of Nanjing, capital of Jiangsu, was diagnosed with the H5N1 strain of bird flu late last week in the eastern province, only days after his 24-year old son died from the deadly virus.
When members of the same family contract the disease alarm bells ring because health experts fear the virus has mutated into a form which is transferred between humans, a scenario which would set the stage for a global pandemic.
Until now all cases of the H5N1 strain of bird flu in humans has been the result of close contact with infected poultry or birds.
Chinese Health Ministry spokesman Mao Qun'an says an analysis of a sample taken from the dead son has indicated the bird flu virus has not mutated, but the possibility of human-to-human transmission in this case could not be excluded.
Mao Qun'an says the virus remains an avian virus and has not undergone a mutation.
He speculates that one of the men might nonetheless have infected the other through close contact, or they might have become infected from another source or even from separate sources.
Mao Qun'an says a thorough investigation is under way and until that is complete a final determination on these three possibilities cannot be offered.
The father is now said to be in stable condition and showing signs of improvement and of the 69 people who had close contact with the son, 55 have been released from medical observation.
Among the 20 close contacts with the father, six also had close contact with the son and are currently under strict medical observation, but so far none have shown any unusual symptoms.