Actor Charlie Sheen's announcement that he's been HIV positive for four years could help to clear up some public misconceptions and stigma, experts say.

The star of Two and a Half Men and Anger Management is one of about 1.2 million people in the United States, 71,300 in Canada and 35 million globally estimated to be living with HIV.

About 25 per cent of them were not aware of their infection, because of a lack of testing or diagnosis, according to the Public Health Agency of Canada's website.

Sheen's doctor, Robert Huizenga, appeared with the actor on the Today show Tuesday and said Sheen "does not have AIDS." Huizenga said later in an open letter released by the actor that Sheen was taking four antiretroviral drugs and that his HIV viral load was now "consistently undetectable."

Says he always uses condoms

When Sheen was asked whether he'd transmitted HIV to anyone, he said it was "impossible," adding that he had "always led with condoms and honesty when it came to my condition."

An undetectable viral load means the amount of HIV particles in the blood is below the level that can be detected by laboratory tests.

While it's unlikely to pass on HIV when the viral load is undetectable and protection is used, doctors say a small risk remains.

Sheen also released a statement on how the secrecy opened him up to extortion.

"The personal disbelief, karmic confusion, shame and anger led to a temporary yet abysmal descent into profound substance abuse and fathomless drinking. It was a suicide run. Problem was, I'd forgotten that I'm too tough for such a cowardly departure. Yet, despite this loathsome and horrific odyssey, I was vigilant with my anti-viral program," Sheen wrote in part.

With the introduction of antiretroviral medication for HIV, people with HIV/AIDS in high-income countries can expect to live a normal lifespan.

Sheen said he wants to use the opportunity to help others. "My partying days are behind me. My philanthropic days are ahead of me."

Huizenga said his biggest concern with Sheen is "substance abuse and depression from the disease," not HIV shortening his life.

'Bigotry and hatred'

Dr. Philip Berger, the medical director at the Inner City Health Program at St. Michael's Hospital in Toronto, started treating people infected with HIV more than 30 years ago. Berger hopes Sheen's story might help with the lingering stigma.  

"Having HIV is not yet free of bigotry and hatred by other people, even though it is much, much less than 30 years ago when Rock Hudson first declared his HIV status," Berger said.

David Stempowski, who is with the Toronto People With AIDS Foundation, was diagnosed with HIV in 2011. It took him six months to tell those close to him. Stempowski takes antiretrovirals and his viral load is considered undetectable.

Stempowski is also the acting chair for the Canadian AIDS Society in Toronto, where he works to end stigma and discrimination against HIV and AIDS.

Stempowski said it must have been "hell" for Sheen trying to keep his status a secret. He respects Sheen for disclosing his status publicly now, given that his career was based on upholding an image.

"He had a certain persona to live up to. I mean he was the highest-paid actor on TV at one point," said Stempowski. "He's a straight guy … I mean who was the last straight guy that's come out as HIV positive? Magic Johnson?"

Fear of being judged both in the workplace and by potential partners continues to exist, he said. 

With files from CBC's Vik Adhopia and The Associated Press