Emilian David Armstrong says in the suit ... he met Barker in early 1996, and they quickly moved in together. Armstrong had a daughter from a previous heterosexual relationship. She also moved in.

Armstrong says in November, 1996, he was diagnosed with HIV. The suit claims Barker had tested positive and had urged Armstrong to get tested. Armstrong is convinced -- Barker gave him HIV.

Armstrong says Barker confessed he had previously engaged in a sexual relationship with his own cousin, who had AIDS -- and also engaged in sadomasochism in prior relationships that involved syringes.

As for why Armstrong waited 16 years to file a lawsuit -- Armstrong says he and Barker tried to work out their relationship and battle the virus together ... but things started to go south in 2003.

According to the lawsuit, that's when Barker became addicted to prescription painkillers such as Vicodin, Percocet, Demerol, and Oxycodone. Armstrong claims Barker would mix these drugs with alcohol, cocaine, and/or crystal meth and use them for recreation.

But Armstrong says he still didn't give up on the relationship -- and tried to help Barker through his drug problems ... but they only got worse. Armstrong says Barker started throwing drug-fueled parties with young men ranging in age from 19 to early 20's.

The suit claims in 2009, Barker kicked Armstrong out of his house -- along with Armstrong's daughter -- and left them destitute.