May 23 (Bloomberg) -- United Nations disease trackers probing the possibility of human-to-human transmission of bird flu said a group of seven Indonesians may have contracted the disease from other people rather than birds.

All the people in the cluster of cases had contact with severely ill patients, the World Health Organization said in a statement today. Six of the seven people have died, and a team of international experts has been unable to find animals that could have been the source of the virus, the agency said.

A 10-year-old boy in the extended family may have caught the virus from his aunt and passed it to his father, the first time officials have seen evidence of a three-person chain of infection, an agency spokeswoman said. Strong evidence might prompt the global health agency to convene a panel of experts and consider raising the pandemic alert level, said Maria Cheng, an agency spokeswoman.

``Considering the evidence and the size of the cluster, it's a possibility,'' Cheng said in a telephone interview. ``It depends on what we're dealing with in Indonesia. It's an evolving situation.''

Almost all of the cases of H5N1 infections confirmed by the WHO since late 2003 can be traced to direct contact with sick or dead birds.

Lethal to People

Fear of a human spread of the virus is running high in part because it is so lethal to people, who have no natural immunity to it. The virus has killed two-thirds of those confirmed to be infected this year. Since late 2003, the virus has sickened 218 people in 10 countries, killing 123 of them, WHO said today.

The 32-year-old father in the cluster of cases on the island of Sumatra was ``closely involved in caring for his son, and this contact is considered a possible source of infection,'' The WHO said in its statement. Three others, including the sole survivor in the group, spent a night in a ``small'' room with the boy's aunt, who later died and was buried before health officials could conduct tests for the H5N1 virus.

``All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness,'' the WHO said.

While investigators have been unable to rule out human-to- human transmission in the Sumatran cluster, they continue to search for other explanations for how the infections arose, the WHO statement said.

Genetic Changes

Health experts are concerned that if H5N1 gains the ability to spread easily among people, it may set off a lethal global outbreak of flu. While some flu pandemics are relatively mild, the 1918 Spanish flu pandemic killed an estimated 50 million people worldwide.

So far, studies of the Sumatran outbreak and genetic analyses of the virus don't indicate the virus has undergone major changes, Cheng said. Scientists at WHO-affiliated labs in the U.S. and Hong Kong found no evidence that the Indonesian strain of H5N1 has gained genes from pigs or humans that might change its power or spreading ability, WHO said.

``These viruses mutate all the time and it's difficult to know what the mutations mean,'' Cheng said.

Health officials earlier found strong evidence of direct human-to-human spread of H5N1 in Thailand in 2004. Scientists reported in the Jan. 27, 2005, issue of the New England Journal of Medicine that the H5N1 virus probably spread from an 11-year- old girl in Thailand to her aunt and mother, killing the mother and daughter. People who had more casual contact with the girl didn't become infected.

Contact With Illness

In the Sumatran cluster, close, direct contact with a severely ill person was also needed for spread, Cheng said. Preliminary findings from the investigation indicate that the woman who died, considered to be the initial case, was coughing frequently while the three others spent the night in the same room. One of the three, a second brother, is the sole survivor. The other two, her sons, died.

``It looks like the same behavior pattern'' of close contact and caretaking during illness with the bird flu virus, Cheng said. To raise the level of pandemic alert ``it would have to be transmissible from more casual contact.''

The Indonesian Ministry of Health and international scientists are continuing their investigation to trace the origins of the infections, the WHO said in its statement.

General Community

``Priority is now being given to the search for additional cases of influenza-like illness in other family members, close contacts, and the general community,'' the WHO said. ``To date, the investigation has found no evidence of spread within the general community and no evidence that efficient human-to-human transmission has occurred.''

The WHO's pandemic alert now is at the third of six levels, indicating that a new flu virus subtype is causing disease in humans, though not yet spreading efficiently and sustainably among people. To raise the alert by one level, WHO would convene a panel of outside experts who would make a recommendation to acting Director General Anders Nordstrom, who would order the change.

Nordstrom took over for the late Lee Jong-Wook, who died yesterday after surgery on May 20 to remove a blood clot from his brain.

Each of the six stages in the alert system triggers a series of recommended activities to be undertaken by WHO, other international organizations, governments, and industry.

A WHO planning document that lists events that might cause the level to change includes: moderate or worse lung disease in five to 10 persons with evidence of human-to-human transmission in at least some, and laboratory confirmation of H5N1 infection in at least two patients.

``If we now say we should raise the alert level, we have to justify why this is different,'' said Michael Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy in Minneapolis, in a telephone interview yesterday. ``I'd hate to see the Indonesia cases create a sense of major urgency, if in fact it's more of the same.''

To contact the reporter on this story: John Lauerman in Boston at jlauerman@bloomberg.net.

Last Updated: May 23, 2006 17:02 EDT