Joanna Brent, a Beijing-based WHO spokeswoman, said the father began showing symptoms on Monday and was confirmed as having the virus on Wednesday. She said he has been hospitalized and is being treated.

"Because the possibility of human-to-human transmission cannot be ruled out, we will be monitoring this case closely," Brent told The Associated Press.

"If it is found to be easily passed between humans, we would be concerned," Brent said.

Brent said there was no evidence that the man had been infected by his 24-year-old son, who died on Dec. 2, but said the possibility could not yet be eliminated. Chinese news reports gave the man's age as 52. Both he and his son, who lived in the eastern province of Jiangsu, were identified only by their surname, Lu.

Brent said it was also possible that both men were infected by the same source, or that they were infected separately from different sources.

Jiangsu's Provincial Disease Control and Prevention Center said the son - China's 17th official fatality from bird flu - had not had any known contact with dead poultry, and there were no reported outbreaks of the disease in the province.

Brent said health authorities were monitoring another 68 people who were in close contact with the son, none of whom have so far shown symptoms of H5N1 infection. She said that seemed to indicate that it was unlikely that the virus was being easily passed between humans.

China has not confirmed any cases of human-to-human infection, although the sister of a Chinese boy who was diagnosed with H5N1 in 2005 later became sick and died. Authorities were not able to confirm whether the girl had been infected with H5N1.

Sporadic human-to-human transmission of the highly viral and hard-to-treat H5N1 flu strain has been reported in Hong Kong, Vietnam and Indonesia, but none of the cases have been proven, and officials determined there was no epidemiological significance because the spread was not sustained.

Despite that, Dr. David Nabarro, the U.N. official coordinating the global fight against bird flu, said last month that the risk of a worldwide human-to-human pandemic remains as great today as it was when H5N1 first gained intense media attention in mid-2005.

Bird flu in poultry and wild birds has since spread to 60 nations, but improved responses have limited it mainly to just six nations: Indonesia, parts of Bangladesh, Vietnam, Egypt, Nigeria and China.

Experts say the virus has not been able to commingle its genetic material with that of a human influenza virus and, in so doing, acquire the ability to be transmitted from person to person.

Most people killed by the disease so far have been infected by domestic fowl, and the virus remains very hard for humans to catch; about half the people infected die. But experts fear it could mutate into a form that easily spreads between people, sparking a pandemic that some have said could kill anywhere from 5 million to 150 million.