A mainland man who died of bird flu passed the disease on to his father in the latest human case, the Ministry of Health confirmed yesterday. But ministry spokesman Mao Qunan said there was no evidence that the virus had mutated into a form that was easily transmissible among humans. He said the 52-year-old father in Jiangsu province was infected through close contact with his 24-year-old son, who died last month. 'The initial judgment is that it was an infection from close contact,' Mr Mao said. 'It has no biological features for human-to-human transmission.' He said the father had recovered. However, Mr Mao said experts still could not determine the source of infection for the father and son, as both had no known history of contact with dead poultry or birds - the primary source of H5N1 bird flu. The World Health Organisation said yesterday it would not rule out the possibility of animal-to-human infection in the case and there was no immediate threat of an epidemic because the virus had not mutated into a form that could jump easily between humans. 'A human-to-human transmission through close contact between the son and the father cannot be ruled out in this family cluster,' Hans Troedsson, the WHO representative in China, said in a written answer to questions from the media. 'However, the biological findings at this stage show that the virus has not mutated to a form that can be transmitted from human to human efficiently. 'The source where the son contracted the virus from has not been concluded yet. WHO understands that there is no confirmed contact with sick or dead poultry for either case. However, the possibility of animal-to-human infection cannot be excluded.' He said the organisation would not ask to be allowed to send experts to look into the case for the time being. 'WHO has offered and provided technical support to the Ministry of Health in its investigation and follow-up of this case. Right now there is no immediate need for WHO support,' he said. 'WHO had regular meetings with the Ministry of Health to discuss the two Jiangsu H5N1 cases, and the two agencies will have regular contact in the future.' The young man, surnamed Lu, developed fever, chills and other symptoms on November 24 and was admitted to hospital on November 27 after being diagnosed with pneumonia. His father developed a fever and was admitted to hospital for pneumonia in a lower lobe on December 3, the day after his son's death, according to Xinhua.