At around 4 on a Saturday morning, a time when most of the gay bars in New York have closed and locked their doors, a steady stream of young and middle-aged men, almost all shirtless and some stripped down to their boxer briefs, have found their way down a dark stairwell and into a maze of basement rooms, where the décor can best be described as fallout-shelter chic.

They have come to Paddles, an after-hours sex club in Chelsea, not yet ready to end their evening. They prowl the long cinder-block hallway, exchanging knowing glances. A husky, bearded man in his 40s lounges on a corrugated black rubber bench, admiring a chorus line of smooth-chested 20-somethings, their flesh glowing under a pink neon sign and black lights. A man in a metal-studded black leather chest harness strides toward a back room, the hookup room, where a circle of men, skin glistening with sweat, hover around a swing, watching.

Then, in walks a skinny man in a black baseball cap, with soulful eyes and a nose that juts forward like the prow of a ship. He stops at a folding table set up between two video screens showing continuous reels of gay pornography. He strips off his black leather jacket, flexing toned biceps in a black muscle shirt. He sets up a red hazardous-waste bin as nonchalantly as if it were a plastic juice jug from Costco, arranges some Band-Aids and a bowl of lollipops next to it, and pulls out a syringe.

This is Demetre Daskalakis, a doctor and gay activist who has come to spread the message that a new health threat has emerged among the city’s gay population and that he is there to stop it.