Symptoms include: anything they complain of. Then tell them that anal clap is worse than AIDS, and it's only going to get worse. One could also make up some more heinous symptoms, including (but not limited to) anal leakage, scabies, and loss of teeth.
Simon: "What's anal clap?"
Paul: "The worst STI in the world... Its pretty serious. And by that I mean its worse than AIDS. If your bum gets itchy tonight I'd be worried..."
2. Susan: "I'm just really worried that I might have caught an STI off that guy last week... (insert list of symptoms here)."
Dolores: "Oh dear, Susan. It sounds as though you've caught anal clap!"
Susan: "No! Anal clap is worse than AIDS! I should have known that he'd be infected with the worst STI on the planet!