London, Dec 8: Fears of human-to-human transmission of bird flu have been raised after the father of a Chinese man who died of the infection has also been diagnosed with the virus.

Till now, humans have caught the virus from sick birds but scientists fear if the virus gains the ability to pass easily between humans, tens of millions of people could die.

The father of a 24-year-old man, who died of bird flu on Dec 2 in the eastern province of Jiangsu, tested positive for bird flu after displaying symptoms of the disease.

The World Health Organization said that it could not rule out the possibility that the H5N1 virus had spread from the son to the father.

"We are concerned. The fact that we have two cases here without necessarily a clear source of animal infection and within the same family means we need to make sure we do a thorough investigation, " BBC quoted John Rainford, WHO spokesman, as saying.

Presently, the 52-year-old man is being monitored in hospital.

According to WHO, it is also possible that both men were infected by the same source, or that they were infected separately from different sources.

Suspected human-to-human transmission of the virus has been reported in Hong Kong, Vietnam and Indonesia, but none of the cases have been proven. (ANI)