The condition of the 52-year-old father, who was identified only with his surname, Lu, is now being closely monitored after he showed symptoms of H5N1 on Monday and was confirmed on Wednesday to have the virus.
"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," WHO spokesman John Rainford told the BBC News.
The report also raised the possibility that the men could have been separately infected by different sources or that they could have both been exposed to the same source.
The son died on Dec. 2 after he was earlier admitted in a hospital due to symptoms of pneumonia but later turned out to be positive for the H5N1 strain of avian influenza.
The Jiangsu health department meanwhile said both the father and the son had no direct contact with poultry birds, as it maintained there is no outbreak in the area where the victims caught the virus.
At least 26 bird flu cases have been confirmed across China since 2003, and of the total, 17 deaths were registered.
At present, there is no evidence of efficient human-to-human transmission or of airborne transmission of H5N1 to humans. But in most cases, those infected with the virus had extensive physical contact with infected birds.
