By ANEMONA HARTOCOLLIS

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.

“Have you been vaccinated?” he asks, smiling, his voice warm, as the half-naked men walk by.

A new, casually transmittable infection — a unique strain of bacterial meningitis — has cast a pall over the gay night life and dating scene, with men wondering whether this isAIDS, circa 1981, all over again. Seven men have died in New York City, about a third of diagnosed cases, since 2010. And in the last few months, the contagion seemed to be accelerating. It has targeted gay and bisexual men, and nobody knows exactly why.

The city’s best hope to curb the outbreak is to vaccinate as many at-risk men as possible, focusing on those most in danger: men who regularly hook up with other men whom they meet at parties, bars, clubs and through apps like Grindr. Dr. Don Weiss, the director of surveillance for the city’s Bureau of Communicable Disease, has called it “Russian roulette sex,” because “sooner or later, you are going to come across this organism and be exposed.”

The health department’s own vaccination efforts at several gay bars have had limited success. Men out partying want to have fun, not be told that they may fall prey to a lethal disease by doing so.

Hence Dr. Daskalakis’s early-morning club crawl, medical bag in hand. Being a nonthreatening gay man who does not wear a white coat helps. So does his empathy and sense of humor. When this reporter sent him an e-mail expressing a wish to remain fully clothed while out cruising, the reply from his iPhone was instantaneous: “I will be in a burkha ”

Every half-hour or so, the owner of Paddles, Michael Aulito, makes a public-service announcement: “If you haven’t gotten a shot, please go talk to Dr. Demetre.” Once, he adds, “Not Dr. Demento, Dr. Demetre.”

Dr. Daskalakis hands the men a consent form and asks the threshold question: “Do you have an illness more serious than a cold?”

“Will it hurt?” they ask.

“I’m hitting more than 700 today, my injections have gotten really good,” he says, grinning proudly.

“Can I get it from having sex?”

“Maybe,” he says, “but not just from sex. You can get it from being close, like kissing or cuddling.”

This is motivating. Most of the men have checked their shirts at the door. Some have checked their pants. Conditions for injection could hardly be better. Before Lady Gaga can stutter out “Pa-pa-pa-poker face, pa-pa-poker face,” Dr. Daskalakis stabs them in the arm with a needle, applies a Band-Aid and sends them on their way. All over Paddles, men are happily sucking on the lollipops he is handing out as a reward.

“Dr. Demetre told me every person who gets a shot saves four other people,” Mr. Aulito says. “If he gives 700 shots, that’s 2,800 people that he saved, an amazing number.” Mr. Aulito has been vaccinated, as has his wife.

Peter DeMartino, 40, the head of an AIDS organization, AIDS/HIV Services Group, in Charlottesville, Va., says that he is so excited at finding medical care in a sex club that he feels like waking up his traveling companion, who is from Philadelphia, to get inoculated, too. “It’s New York, right?” Mr. DeMartino says. “We know our populations are very — migratory is not the right word — but it’s not much to have a weekend in New York. If there’s an outbreak in New York, how soon before it’s up and down the 95 corridor?”

Freddie Messina, a 38-year-old events coordinator from Bensonhurst, Brooklyn, said that he got his shot the week before from his doctor, but he is grateful that, for men who are not insured, Paddles is an option. “I go out quite a bit,” he says. “You don’t have to have sex with someone to get meningitis. You go out to a bar on Friday night, you’re in contact with hundreds of men, and you’re not going to know. You’re going to think, ‘Oh, I’m hung over.’ ”

The bearded man lounging on the bench, an investment-bank asset manager who does not want to give his name, says the outbreak has definitely affected his behavior. “I have to hold back on kissing, which I normally do as an alternative to more aggressive or deep intimacy,” he says. He watches others get vaccinated, musing, “This might be a convenient time,” but is soon distracted by a bevy of younger men wandering by, and takes off behind them.

A publicist has been dragged in by his “close to monogamous” boyfriend of 13 years, since they met at N.Y.U. Refusing to take off his jeans, sport coat, button-down shirt and boat shoes, he grabs the consent form as if it were a note excusing him from gym class. He studies it carefully and says he and his companion, who is rushing through the consent form as fast as he can, came to Paddles specifically in search of the vaccine. “We left a bar tonight and we got a flier for this place,” he says. “It had Gay Men’s Health Crisis on it. We had no idea it was an underground sex place.”

He says he is a friend of a friend of one of the men who died, horrifically. “I think he just didn’t wake up,” the publicist says. “I’m like, ‘I’ll take a free shot.’ ”

Meningitis is an inflammation of the lining that surrounds the spinal cord and brain. Symptoms can come on so fast and seem so ordinary — fever, headache, stiff neck and a rash — that victims often try to tough it out and neglect to go to the doctor until it is too late.

The bacteria is carried in the nose and mouth. Though not as contagious as a cold or flu, it can be spread through kissing, sneezing or sharing a spoon. (Sharing cigarettes is also bad, but there is a theory in the literature that this is not because of the exchange of saliva but because smoking irritates the mucous membranes and facilitates bacterial invasion.)

The publicist takes off his jacket and pulls up his shirt just long enough for a shot, then buttons up again.

“How long is it good?” he wants to know.

“It takes 7 to 10 days to take effect and provides up to five years of immunity,” Dr. Daskalakis says.

Often, men ask him, “What are my chances of getting this?”

“Minuscule,” he replies. The idea, he explains, is to confer herd immunity by vaccinating as many at-risk men as possible.

After coming out of the closet and AIDS and the fight over equal rights and gay marriage, they’ve been through enough, some gay men say. “As one of my guys who I vaccinated said, ‘What are they going to think of next?’ ” Dr. Daskalakis commiserates.

“It could have picked another social network,” he says. “It picked gay men. It’s like thinking of the community as a large dorm without walls.”

Knowing that Dr. Daskalakis has entree where government apparatchiks do not, the city supplies him with free vaccine. A week ago, he vaccinated men at a house party in Brooklyn, in a location he did not want to be disclosed, where the host set the mood by dressing in drag as a platinum-blond nurse. “The department of health loves that we’re here — loves, loves, loves,” he says, at Paddles.

So far, the fear is nowhere on par with the AIDS epidemic, which led to fights over closing bathhouses and protests in the streets that the local and national governments were not doing enough. But to a bearded Irishman in his early 40s at Eastern Bloc, the East Village club where the mood is “Cheers” meets hipsters, this moment is reminiscent of when AIDS was pejoratively known as GRID (Gay-Related Immune Deficiency).

“I feel like it’s the next narrative on from H.I.V.,” the Irishman says, before he walks off with his bicycle and a Venezuelan friend. “ ‘Gay something happens in New York City gay spots.’ ”

Another patron, a 21-year-old art student in fashionably square glasses, says he has felt some backlash from straight people. He offered a co-worker a cookie, “and they asked me do I have meningitis.”

But there are important differences. AIDS was considered a death sentence, until antiretroviral drugs were developed to keep the virus in check. Meningitis can be treated withantibiotics if caught in time. The vaccine will prevent someone from getting it, and possibly reduce the ability of a carrier to spread it.

The current strain was first detected among drug users in Brooklyn in 2006. In that outbreak, 23 people were infected and 7 died. After the city conducted a vaccination drive at drug treatment centers and soup kitchens, there was a three-year lull. Then there was 1 case in 2010, 4 in 2011, 13 in 2012 and 4 so far this year, all among gay and bisexual men living in New York City. Of the 22, 7 have died. (City officials say there has been a 23rd case in a man who lived elsewhere in New York State but frequented the city.)

The archetypal case is an African-American man 20 to 40 years old who is socially active and lives in a cluster of Brooklyn neighborhoods: Bedford-Stuyvesant, Brownsville, Bushwick, Clinton Hill, Crown Heights, Downtown Brooklyn, Dumbo, East New York, Prospect Heights and Williamsburg. Half of the men have been black, 18 percent Hispanic. Often, they are not out about their sexual activity, making it difficult for the health department to reach them. There have been 10 cases in Brooklyn, 7 in Manhattan, 2 in Queens and 2 in the Bronx. One man was homeless.

Of the 22, 12 were H.I.V.-positive, a possible risk factor because of their compromised immune systems. The last confirmed case was Feb. 15.

Epidemiologists are puzzled as to why the latest outbreak is attacking men but not women. “We don’t have any evidence that it’s different in some biological way, we just know that it’s different,” says Dr. Jay Varma, the city’s deputy health commissioner for disease control. “And what is concerning is that it is largely restricted to men who have sex with men. So we don’t really understand why that is.”

Last month, the death of Brett Shaad, a West Hollywood, Calif., lawyer, fueled concerns that the outbreak had spread west. Los Angeles County health authorities and the Centers for Disease Control and Prevention have since confirmed that the strain of meningitis that killed Mr. Shaad was different enough from the one in New York that it is unlikely that is how he was exposed. But the C.D.C. has asked state health departments nationwide to be on the alert for cases among gay men.

If meningitis were spread by mere contact, you would expect it to be ripping through No Parking, a gay bar in Washington Heights. After 1 a.m. at a popular Wednesday night party, the men (and some women) are squeezed in shoulder to shoulder. Underwear-clad go-go boys hop down from the bar, flashing their rock-hard backsides. Patrons tuck dollar bills into the dancers’ waistbands. A dancer leads a fully dressed customer behind a curtain for a lap dance. There are communal hookahs, but the bar passes out plastic-wrapped individual mouthpieces.

“I think of it as a safe haven for people to come out and be themselves,” says Romeo Romero, the party promoter, draped in gold chains. “I don’t want to be a party pooper, but we’ll make the announcements: ‘H.I.V. testing around the corner.’ So there are options. I don’t think they’re worried about it that much, although it’s potentially dangerous and harmful, and they should be.”

Indeed, a striking number of the men at No Parking say they have no idea of a meningitis outbreak in New York. “I’m scared right now, I’m shocked, I didn’t know,” says Fabio Reyes, 21, a cook’s assistant, after being told. “He’s shocked, too,” he adds, indicating the man standing next to him, Wilson Columna, 25, a home attendant.

In one positive sign, the number of vaccinations in the city has been rising sharply, to 10,200 as of May 13.

“We have our fingers crossed,” Dr. Varma said. “One of the reasons we can’t be 100 percent confident is that there are a number of Gay Pride events coming up, where there are a lot of people coming into the city, a lot of people interacting together, so we want to get through that period before we are really celebrating controlling it.”

It is now dawn on Saturday at Paddles, though you would not know it from the darkness underground. The asset manager shambles out of one of the back rooms. “Somebody told me my breath is too fresh,” he says, mock insulted. He blinks at the lithe man in the black muscle shirt, still holding a syringe, as if seeing him for the first time. “So you’re a doctor?” he says, and steps up for his shot.