Former "Two and a Half Men" star Charlie Sheen has HIV. In an interview Tuesday on NBC's "Today," the 50-year-old actor said he tested positive for the virus that causes AIDS about four years ago, but that, thanks to a rigorous drug regimen, he's healthy.

When asked by co-host Matt Lauer if he had transmitted the disease to others, he declared, "Impossible. Impossible," and insisted he had informed every sexual partner of his condition beforehand.

Since his diagnosis, he has had unprotected sex with only two partners, he said, adding, with no further clarification, "They were under the care of my doctor and they were completely warned ahead of time."

He said one reason for going public with his condition was to put a stop to shakedowns from prostitutes and others who threatened to out him. He said one prostitute took a photo of the HIV-related drugs in his medicine cabinet and threatened to sell that photo to the tabloids.

He said he had paid "enough to bring it into the millions" perhaps as much as $10 million to buy their silence and now was seeking to "put a stop to this barrage of attacks and sub-truths."

"Are you still paying these people?" Lauer asked him.

"Not after today I'm not," said Sheen, who during his appearance appeared jumpy and spoke in stuttering bursts.

He did not address when, and by whom, he may have been infected.

In the U.S., AIDS is spread mainly through having sex or sharing injection-drug equipment with someone who has HIV. About 1.2 million people in the U.S. are estimated to have HIV, with many of them unaware.

For part of the interview, Sheen was joined by his physician, Dr. Robert Huizenga, who said strong anti-viral drugs have suppressed the virus. The HIV level in Sheen's blood is "undetectable," Huizenga said, and flatly stated that he does not have AIDS.

"He is absolutely healthy," said Huizenga, adding that his own concern was for Sheen's "substance abuse and depression from the disease more than what the HIV virus could do to shorten his life, because it's not going to."

Sheen said in the past that he was "so depressed by the condition I was in that I was doing a lot of drugs, I was drinking way too much." He said he currently is not abusing drugs, though he allowed that he is "still drinking a little bit."

"My partying days are behind me," Sheen said in an "open letter" released by email. "My philanthropic days are ahead of me."

Asked by Lauer if he expected "a barrage of lawsuits" from past sexual partners alleging he infected them, he said wanly, "I'm sure that's next."

But in California, where Sheen resides, a person can be charged with a felony only if they are aware they are HIV-positive and engage in unprotected sex with another person with the specific intent of exposing them to the disease.

Sheen could potentially be sued in civil court by any partners he hasn't already settled with, with the amount of damages they would recoup determined by a judge or jury.

With his public pronouncement, Sheen said he hoped to reduce the stigma and shame still felt by some others diagnosed with HIV, as well as by those who are reluctant to be tested in the first place.

"I have a responsibility now to better myself and to help a lot of other people," he said. "And hopefully with what we're doing today, others may come forward and say, 'Thanks, Charlie, for kicking the door open.'"

The disclosure was the latest chapter in Sheen's headline-seizing history. In recent years, drug and alcohol abuse led to his being kicked off CBS' "Two and a Half Men" in 2011 after an explosive meltdown that included calling the show's producer "a contaminated little maggot."

His escapades also included the revelation that he spent more than $50,000 as a client of "Hollywood Madam" Heidi Fleiss' prostitution ring.

Sheen had become an instant film star in his early 20s with two dramas, "Platoon" (1986) and "Wall Street" (1987), and found success with big-screen comedies as well, including "Major League" (1989) and "Hot Shots!" (1991).

In 2003, "Two and a Half Men" debuted on CBS and starred Sheen as womanizing bachelor Charlie Harper. It made Sheen one of TV's highest-paid actors and at its peak was TV's most-watched sitcom. It ended its run earlier this year, with Ashton Kutcher stepping in for the fired Sheen.

In 2012, Sheen returned to TV in "Anger Management," an FX sitcom adapted from the movie of the same name.

Sheen has been wed three times, the first time to model Donna Peele in the 1990s.

He and actress Denise Richards were married from 2002 to 2006 and have two daughters. Sheen and real estate investor Brooke Mueller wed in 2008 and divorced in 2011; they have two sons.

"Brooke has been inundated with calls from friends and family. To put their minds at ease, Brooke can confirm that she and the boys are not HIV positive," Steve Honig, a spokesperson for Mueller, said in an email.

The Centers for Disease Control and Prevention recommends that everyone between the ages of 13 and 64 have an HIV test at least once as part of routine health care, and that people seek out testing if they have such risk factors as having had sex with someone whose HIV status they didn't know.

Condoms are the best way to prevent sexual transmission.