Two male adult film actors tested positive for HIV this week, with officials speculating that one actor infected the other with the virus while having unprotected sex on a film shoot in Nevada. The incident marks the first time since 2004 that adult film performers contracted the virus on a film set.

In a statement, the California Department of Public Health said that one of the actors tested negative for the virus before a series of recent shoots, but began to show symptoms of being ill when he began work on his second shoot.

“In this case, the actor and production company thought he was HIV-negative during filming,” the statement read, according to the Associated Press. “Shortly after his negative test, HIV levels in his body rose rapidly to where he could infect other actors through unprotected sex.”

The CDPH and adult film industry trade group The Free Speech Coalition issued a moratorium on all porn film shoots immediately after the infections were first discovered. The trade group said that the infections were contracted in September on a Nevada film set that uses sub-standard HIV testing.

“Not only did this leave those who participated at risk, it made it much harder to track scene partners once the possible infection was discovered,” the group said in a statement.

Both California and Nevada do not require HIV testing for adult film performers, AIDS Healthcare Foundation president Michael Weinstein told the Associated Press, although industry rules demand the tests.

“The big lie the industry has been saying all these years, there are no on-set transmissions, has proven to be untrue,” Weinstein told the AP.

Most adult film productions moved out of Los Angeles County after a 2012 law mandated the use of condoms on film sets. According to the AP, the number of adult film shoot permit applications dropped from 485 in 2012 to just 40 in 2013. The Free Speech Coalition said in the report that “non-compliant shoots are one of the chief dangers” of the adult industry’s move out of California.

The Centers for Disease Control and Prevention reportedly conducted genetic testing to determine whether the actors contracted the virus on set, and found a “very high match” that they had. Other actors on the Nevada set were reportedly offered and accepted HIV testing after the initial viruses were discovered.