Eurovision winner Conchita Wurst says she is HIV positive, and has been receiving regular treatment for "several years".
The 29-year-old star said she had intended to keep the information private, but an ex-boyfriend had "threatened to go public".
"I will not give anyone the right to frighten me and influence my life," she wrote on Instagram.
"Coming out is better than being outed by a third party."
Wurst added that regular treatment had suppressed the virus to the extent it could no longer be detected in her blood - and could no longer be passed on.
"I hope to give others courage and to take another step against the stigmatization of people who, through their own behaviour or that of others, have become infected with HIV," she added.
Aids charity The Terrence Higgins Trust expressed dismay that Wurst had been forced to reveal her diagnosis.
"The decision to talk openly about your HIV status should be a personal one and not taken away or ever, ever used as a threat," chief executive Ian Green said.
"Threatening to reveal someone's HIV status, under any circumstances, is entirely wrong. What other health condition would be used as blackmail against someone? And we know this isn't something which only happens to those in the public eye."
However, he praised Wurst for handling the situation "with such dignity" and attempting to "tackle the abhorrent stigma" surrounding HIV and Aids.
