The story will unfold after Set has unprotected sex with a young man named Connor, who is HIV positive but unaware of his status.
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Following the one night stand, Connor is diagnosed and is forced to break the news to Set, who then has to decided whether or not to take a HIV test himself.
Sadly for Kieron Richardson's character, it will not be good news - and, after his diagnosis in January, the soap will focus on the effect that it has on his relationship with John Paul McQueen, his children and his physical and psychological wellbeing.
Hollyoaks' executive producer Bryan Kirkwood said: "We have wanted to tell this story for a long time and while HIV can affect anyone, infection rates in young gay men remain too high and to ignore that is to do the gay audience a disservice."
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He continued: "Hollyoaks is in a unique position to be able to talk directly to millions of young viewers and if the safe-sex message is not coming through education, we can help with that both on screen and through multi-platform support.
"2015 is Hollyoaks' 20th year and to make that our 'year of safe sex' felt right for a show built upon telling rites of passage stories. With Ste's HIV and other stories for our teenage characters, we will explore the many implications of unprotected sex and hope to encourage thoughtful debate amongst our audience."
Will Harris, who works at HIV and sexual health charity, Terrence Higgins Trust, added: "Hollyoaks is sending such an important message, not just for young gay men but for anyone who may not have an up to date understanding of HIV, and we think they can do a lot of good by telling it."