A poultry farm worker in Indonesia has become the country's first confirmed victim of the potentially deadly bird flu virus. The discovery makes Indonesia the fourth country in which the virus has crossed from animals to humans since the latest outbreak in 2003.

The H5N1 strain of avian virus has killed at least 55 people in three different Southeast Asian countries since January of 2003, but the case discovered on the island of Sulawesi is the first to be found in Indonesia.

The case was discovered after poultry on the farm where the victim worked became infected. Tests later showed that the man was carrying antibodies for the virus. Although the man has shown no symptoms and is no longer infected, the presence of antibodies means he was at one time a carrier of the disease.

The acting World Health Organization representative in Indonesia, Dr. Frits De Haan, said the report of the infection came as no great surprise. "This is not completely unexpected. I mean this has been happening in Vietnam and China and Thailand. Antibodies - it is just an indication that the virus is around and that humans can produce antibodies against it," he said.

But Mr. de Haan said governments and health officials have to remain vigilant, in order to head off the spread of the virus before it turns into a serious epidemic.

Indonesia implemented a massive vaccination program for domesticated chickens, ducks and quail after the virus was detected here two-years ago, but the disease has continued to spread among birds. Earlier this year, epidemiologists confirmed that the virus had also spread to some pigs.

The H5N1 virus first appeared in Hong Kong in 1997, and killed six out of 18 people who were infected. Since its re-emergence, it has killed people in Vietnam, Thailand, and Cambodia.

Its spread from animals to humans has been limited, but scientists are worried the virus will mutate into a form that is easily transmissible from person to person.