One person caught the bird flu virus while working at a contaminated poultry farm during an outbreak in Japan earlier this year, in the first case of human infection confirmed in Japan, the government said Wednesday.

The Health Ministry said blood samples taken from five Japanese earlier this year were tested for avian influenza, and one had antibodies for the disease, confirming the infection. The four others were likely to have been infected but tests weren´t conclusive.

It was the first confirmed case of human infection from the disease in Japan, the ministry said in a statement.

Bird flu has swept through farms across Asia this year, forcing officials to cull more than 100 million birds. The disease also jumped to humans, killing 12 people in Thailand and 20 in Vietnam.

Although there has been no evidence of human-to-human transmission, experts worry that the virus could mutate into a version easily spread among people, thus sparking a global pandemic.

The case confirmed in Japan was of an an employee at Asada Nosan Co.´s Funai Farm in western Kyoto prefecture (state), at the center of Japan´s outbreak in February.

He had a headache that lasted for several days but showed no other symptoms after spending a month spraying disinfectant around the farm from late February, the ministry said.

After the farm was disinfected, the Kyoto government took blood samples from about 86 people, including farm employees, firefighters and local government employees who took part in the operation.

The samples were examined by the National Institute of Infections Diseases in Tokyo.

On Friday, the government said blood samples from five people tested positive for the virus. At the time, officials were unable to determine if any of them had actually contracted the disease.

It wasn´t clear how the five were exposed to the virus. But one said he hadn´t worn a mask at the farm even after the virus was discovered there, the ministry said Wednesday.

Tests showed that it was "highly possible" the other four _ three farm workers and a city inspector _ also had been infected, though none of them became ill, the ministry said.

The operator of Asada Nosan Co.´s Funai Farm, about 370 kilometers (230 miles) west of Tokyo, was given a one-year suspended sentence by the Kyoto District Court for violating livestock hygiene laws and concealing from authorities that the disease was killing chickens at the farm.

Asada´s parents, who also ran the farm, committed suicide because of the scandal.