I know someone that is knowingly spreading aids. what to do?

Actually its a friend of a friend that i know. Never met the guy but his grandmother had admitted to me her grandson has aids, and he has girlfriends that he refuses to tell. The guy with aids has mental problems a is constantly in trouble with cops. He has no value for his or anyone else's life. I need to know who to call so i can report this, or is there anything i can do?