LOS ANGELES, Calif. - Two adult film actors have contracted HIV, with one most likely infecting the other during unprotected sex at a film shoot in Nevada, California where testing was less stringent than industry standards, officials said.

One of the actors had previously tested negative for the virus that causes AIDS before a pair of film shoots, but then began showing symptoms during the second shoot and was later found to be HIV-positive, the California Department of Public Health said Monday in a statement.

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

The infections came amid a major decline in porn filming in Los Angeles County — once the centre of porn production in the U.S. — after the 2012 passage of a law requiring porn actors to use condoms during filming. The number of porn filmmakers applying for shooting permits in the county declined from 485 in 2012 to 40 in 2013.

The Free Speech Coalition, a California-based trade group for the adult film industry, said the pair of film shoots linked to the latest infections occurred in September on a Nevada set using tests that do not detect HIV as early as tests done on sets that fully comply with industry standards.

"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 coalition group said in a statement.

The group said it joined the California Department of Public Health in declaring a production moratorium when the infections were first discovered. It said performers on sets that complied with the more stringent protocols were tested and the moratorium was lifted.

Michael Weinstein, president of the AIDS Healthcare Foundation, said infections have happened before during porn productions and will likely happen again. "The big lie the industry has been saying all these years, there are no on-set transmissions, has been proven to be untrue," he said.

The states of Nevada and California do not require porn actors to be tested for HIV, said Weinstein, who has pushed for testing laws in both states and helped pass the condom law in Los Angeles County. Testing is required under industry rules.

The Free Speech Coalition said regulations such as the condom law drive filmmakers to locations where they don't undergo as much scrutiny by the industry. "Non-compliant shoots are one of the chief dangers of pushing the adult industry" out of California, the group's statement said.

A spokesman for the California Department of Public Health said that in addition to the HIV transmission on set between two actors, two off-set partners have tested positive for the virus as well. One of those off-set partners is believed to have first transmitted the virus to an actor. One of the actors later passed the virus along to another off-set partner.

Genetic testing by the Centers for Disease Control and Prevention showed a "very high match" indicating transmission had taken place between the two actors on set. Both actors also independently denied having off-set sex with each other or the same third person.

Health officials said other actors on the set have been notified of an HIV exposure and have been offered and accepted testing for the virus.

The industry has declared several production moratoriums in recent years, but most were for infections believed to have occurred in the private lives of actors rather than during film shoots.

The last confirmed on-set HIV infection was in 2004. After that, the porn industry adopted monthly testing for a range of sexually transmitted diseases. Last year, the industry increased testing to every 14 days after a woman contracted HIV.

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Associated Press Writer Daisy Nguyen contributed to this report.

News from © The Associated Press, 2014