The Department of Public Health said the male actor tested positive for the virus that causes AIDS after engaging in unprotected sex with several other male actors during two separate film shoots. He had tested negative before the shoot.
"During the second film shoot, he had symptoms of a viral infection," the alert states. "The actor went to a clinic and had another blood test that showed he had recently become infected with HIV."
One actor from the second shoot has since tested positive for HIV. According to the health department, lab results indicate the first actor who tested positive "probably transmitted" HIV to the second.
A health department official was unable to immediately release further details regarding the lab testing and investigation. The alert notes that very early in an HIV infection, the test can be negative "even though the actor really does have HIV."
"In this case, the actor and production company thought he was HIV-negative during filming," the alert states. "Shortly after his negative test, HIV levels in his body rose rapidly to where he could infect other actors through unprotected sex.
A California-based trade group for the adult film industry declined to comment.
A health department official declined to release any information regarding when the transmission had taken place or which company it involved, citing privacy restrictions, but said the apparent transmission occurred in Nevada.
"It's happened before, it's happened now, and it will happen in the future," said Michael Weinstein, president of the Los Angeles-based AIDS Healthcare Foundation. "The big lie the industry has been saying all these years, there are no on-set transmissions, has been proven to be untrue."
The foundation championed an ordinance adopted by Los Angeles County voters in 2012 requiring actors in pornographic films to use condoms. The porn industry has fought the ordinance, saying having actors use condoms would interfere with a film's fantasy element by subjecting viewers to real-world concerns like pregnancy and sexually transmitted diseases.
A federal appeals court recently ruled the ordinance does not violate First Amendment rights.
Several porn companies have moved shoots out of the county over the last two years in response to the ordinance. The number of porn filmmakers applying for permits in LA County has declined sharply, from 485 in 2012 to 40 in 2013.
The last confirmed on-set HIV infection was in 2004. After that, the porn industry adopted monthly testing for a range of STDs. Last year, the industry increased testing to every 14 days after a woman who performs under the name Cameron Bay contracted HIV. Her diagnosis triggered a moratorium on adult film production until all performers who worked with her were medically cleared.