The possible past sufferers, mostly chicken farm workers, came from two prefectures, Saitama and Ibaraki, outside of Tokyo, a Health Ministry official said on condition of anonymity, citing internal policy.
Those who may have been infected showed evidence of antibodies to the H5N2 virus, which is weaker than the deadly H5N1 form, the official said. No virus had been found among them.
No human infection of the milder strain had been reported, and the latest results showed a possibility of bird-to-human transmission of the H5N2 virus for the first time, the ministry official said. The absence of flu-like symptoms among them, however, means the there is little chance of developing a full-blown disease from it, she said.
None of them showed signs of the disease and there was no danger that they would or infect others, she said, but added that the ministry planned to follow up on their health conditions.
In related developments:
In Japan, seventy people have tested positive for antibodies to the H5N2 in Ibaraki, about 65 miles north of Tokyo, and seven others in Saitama, just outside the capital, have tested positive for the same antibodies, the ministry official said.
Saitama health official Tatemitsu Yoshida said the central government notified prefectural officials that the results do not pose any serious problems, but poultry farm workers should take appropriate measures such as wearing masks and washing their hands frequently.
The much more deadly H5N1 virus has killed at least 76 people worldwide since 2003, according to the World Health Organization's Web page.
Japan so far has suffered one case of human infection of the more deadly virus, but no deaths.
There have been several outbreaks of bird flu among poultry flocks in the two prefectures, mostly in Ibaraki since last year. A total of 353 chicken farm employees at affected farms as well as quarantine officials have undergone various tests since the outbreak started in June, the ministry official said.
Japan has culled hundreds of thousands of birds to stop the disease's spread since it was detected in the country in 2004 for the first time in decades.
Ibaraki alone had culled 2.5 million birds since the outbreak began, prefectural health official Masahiko Shimada said.
Most of the human infections in the world have been linked to direct contact with sick poultry, but scientists fear that the virus could mutate into a form that spreads easily between humans, sparking a global flu pandemic that could kill millions.
There is no known cure or vaccination for H5N1 in humans.