The patients are being closely monitored for two weeks to ensure that the epidemic will be contained.
Public Health Ministry Inspector General Kamron Chaisiri said the infections were found in the Nakhon Ratchasima facility in the northeast in early June.
Six female and 35 male patients have been admitted, including six who are hospital staff, he said.
According to the initial investigation, one hospital worker was infected with the flu virus from his daughter who fell sick on June 8, with all members of the family subsequently infected with the disease.
The hospital detected the first flu victim on June 12 in the ward for alcoholism patients. The ministry official said that such patients have weaker immune systems and are easily infected with disease.
The patients were given the antiviral drug Tamilflu, and none have severe condition, he said, adding that the patients were separated for better treatment and care.
Mr Kamron said the hospital has temporarily stopped admitting new patients until the situation is confirmed safe in two weeks but outpatient services are operating normally.
Health officials have sampled the schools in the province and have discovered no other cases of bird flu infection.
Report by : MCOT