A Chinese health ministry statement said a 52-year-old man named Lu in the eastern city of Nanjing had the H5N1 strain, which killed his son on Sunday and has reportedly caused more than 200 deaths worldwide since 2003.

The WHO in Geneva said there were three possible explanations for the father-son case: either they were infected by the same animal, by transmission between them, or by exposure to two different infected animals.

The deadly H5N1 strain has passed from human to human only in very rare cases but scientists fear that such transmission could become more efficient and widespread through mutation, causing a global pandemic.

Jiangsu province health department said this week that it had not determined how the dead man contracted the virus as he was not known to have had contact with dead poultry.

The new case brings to at least 27 the number of infections in China, where 17 people have died from bird flu.

"The patient, a 52-year-old male surnamed Lu from Nanjing in Jiangsu, is the father of the serious case of bird flu diagnosed on December 2," said the statement posted on the health ministry website.

"On December 6, the China Disease Prevention and Control Center confirmed the presence of the H5N1 bird flu strain."

The man developed a fever and pneumonia symptoms on Monday while under medical observation following his son's death, the statement said.

The ministry added that all people who had been in contact with the older man were under observation but no new cases had appeared, and that it had promptly notified the World Health Organisation.

WHO spokesman John Rainford said cases of human-to-human transmission are very rare, citing only three previous cases in Vietnam, Cambodia and Indonesia.

Another WHO official, Christiane McNab, said 69 people had been in contact with Lu, and none seemed to have bird flu.

"If this is a case of contact between humans, the virus isn't virulent, otherwise other people would have been infected," she said.

Lu's son, 24, was hospitalised 10 days ago after developing pneumonia, Xinhua news agency reported at the time, citing Jiangsu health department. His condition deteriorated in hospital and he died on Sunday, Xinhua said.

The Chinese health ministry gave no further details on the condition of the new patient, whether he had had contact with poultry, nor any information on possible human-to-human transmission.

While the disease is usually associated with contact with infected birds, only one confirmed human case in China has followed a poultry outbreak.

China conducted a huge campaign last year to contain the disease, slaughtering tens of thousands of fowl.

Vice Agriculture Minister Yin Chengjie, however, warned this September that much of the country remained ill-equipped to prevent its spread.

The H5N1 strain has killed more than 200 people worldwide since 2003, according to the WHO.

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