WASHINGTON (Reuters) - A 24-year-old Chinese man who diedof bird flu in December passed the virus directly to his fatherin a rare case of human-to-human transmission of the virus,doctors reported on Monday.

Chinese officials had already said they believed theyounger man infected his 52-year-old father, who survived, butgenetic sequencing and other checks confirmed this was likely,the researchers said.

"In this family cluster of confirmed cases of infectionwith highly pathogenic avian influenza A (H5N1) virus inmainland China, we believe that the index case transmitted H5N1virus to his father while his father cared for him in thehospital," they wrote in the Lancet medical journal.

H5N1 avian influenza is regularly breaking out in birdsacross Asia, the Middle East, Africa and Europe. It only rarelyinfects humans but has killed 238 out of 376 people known tohave been infected since 2003.

Most have been directly infected by a sick bird, but in afew rare cases, one person appears to have infected another.These have been documented in Indonesia and, just last month,between two brothers in Pakistan.

Most have been among people who are genetically related andthis also appears to be the case with the two Chinese men, theresearchers said. Some experts believe there may be a geneticsusceptibility to H5N1 infection.

The fear is that the virus will acquire changes that allowit to be passed from one person to another more easily. Thiscould cause a pandemic that could kill tens of millions ofpeople globally, so experts are carefully studying every caseof transmission.

Yu Wang of the Chinese Centre for Disease Control andPrevention in Beijing and colleagues investigated the cases ofthe man and his son, who were diagnosed within a week of eachother in December 2007 in Jiangsu Province.

They also tested 91 people the two men had come into closecontact with. None of these people became infected.

The young man had a high fever, cough and watery diarrheaand his father nursed him intensively in the hospital.

The younger man died but his father got the flu drugsTamiflu and rimantadine as well as serum from a womaninoculated with an experimental H5N1 vaccine and recovered.

"With the exception of occasional infection in healthworkers, all published incidents of possible or probableperson-to-person transmission report transmission betweengenetically related individuals," Nguyen Tran Hien of Vietnam'sNational Institute of Hygiene and Epidemiology in Hanoi, andcolleagues wrote in a commentary.

"Although this finding could be related to the intensityand intimacy of contact between family members, host geneticfactors might also play a part in susceptibility to H5N1," theyadded. So anyone in close, prolonged contact with an H5N1victim should get flu drugs just in case, they said.

Last week the World Health Organization said somehuman-to-human spread likely occurred when three brothers inPakistan became infected with H5N1 last year.

The largest known cluster of human bird flu cases occurredin May 2006 in Indonesia when at least 7 family members died.

(Editing by Philip Barbara)