Three years after he abruptly left Big Brother 13, reality star Dick Donato says he left the show because he learned he's HIV-positive.

The 51-year-old Donato won Big Brother 8 and returned for season 13 as part of a team with his daughter Daniele Donato, but he left during the first week. At the time he cited “personal reasons.”

On Wednesday night's episode of cabler VH1's Couples Therapy, Donato said he left the show after learning from its doctors that he was HIV-positive.

“When they told me, I just went numb,” he told PEOPLE.

“My biggest fear was telling my girlfriend. We had this long discussion, and I figured she would freak out. But she was really understanding about the whole thing. She said, 'Whatever happens, we'll face it together.' She was tested and it came back negative. It was an incredible relief,” he added.

Donato said the diagnosis surprised him.

“People are going to make assumptions about how I got it, and that's okay,” he said. “People are afraid to come forward because they're afraid of the stigma of HIV. I'm not gay and I've never stuck a needle in my arm, but at this point, it doesn't matter. We created a stigma around the disease that makes it hard for people to publicly say they have it.”

“When people asked why I left Big Brother, I would give quippy, smart-ass answers,” he said.

Donato said that he's healthy, he only practices safe sex and his viral load is “virtually undetectable.”