It wasn’t his first broken condom, so Rafael didn’t worry. But three weeks later, the man he’d met in a bar called to say that he had “probably been exposed” to H.I.V.

Rafael, a muscular, affable 43-year-old, went to a clinic and within 45 minutes learned he was infected. Although it was already closing time, a counselor saw him immediately and offered him a doctor’s appointment the next day.

At Ward 86, the famous H.I.V. unit at San Francisco General Hospital, the doctor handed him pills for five days and a prescription for more. Because he was between jobs, she introduced him to a counselor who helped him file for public health insurance covering his $30,000-a-year treatment.

“They were very reassuring and very helpful,” said Rafael, who, like several other men interviewed for this article, spoke on condition that only his first name be used to protect his privacy. “They gave me the beautiful opportunity to just concentrate on my health.”

Despite bad luck in sex-with-strangers roulette, Rafael did have some good fortune: He lives in San Francisco, which is turning the tide against H.I.V. and serving as a model for other cities. The city that was once the epidemic’s ground zero now has only a few hundred new cases a year, the result of a raft of creative programs that have sent infection rates plummeting.

“I love the San Francisco model,” said Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases. “If it keeps doing what it is doing, I have a strong feeling that they will be successful at ending the epidemic as we know it. Not every last case — we’ll never get there — but the overall epidemic. And then there’s no excuse for everyone not doing it.”

Last week, the World Health Organization essentially agreed. Issuing new guidelines for the treatment and prevention of H.I.V., the agency called on the rest of the world to do much of what San Francisco is already doing: Every H.I.V. patient should start antiretroviral drugs as soon as they test positive rather than waiting for measures of immune system strength to drop, the agency said, and everyone at risk of infection should be offered preventive drugs. San Francisco adopted the first practice — “test and treat” — five years ago and the second in 2013. It has bolstered those efforts with others, like the rapid-doctor’s-appointment program that swept Rafael in, and another to track difficult patients.

The results have been striking. Last year, San Francisco had only 302 new H.I.V. diagnoses, the lowest recorded number. In 1992, at the epidemic’s peak, there were 2,332.

In 1992, the city had 1,641 deaths from AIDS. Last year, just 177 San Franciscans with H.I.V. died, and most of them actually succumbed to heart disease, cancer or other old-age ills, said Dr. Susan Buchbinder, the head of H.I.V. prevention research for the city health department.

By other measures, too, San Francisco is ahead. According to a 2012 estimate by the Centers for Disease Control and Prevention, only 39 percent of all infected Americans had seen an H.I.V. doctor, and only 30 percent were taking their pills consistently enough to be “virally suppressed,” meaning they were not infectious. By contrast, in San Francisco, 82 percent of residents with H.I.V. were in care, and 72 percent were suppressed. In Ward 86 — all of whose patients are uninsured or on public assistance — suppression has reached 84 percent.

San Francisco has some natural advantages: wealth, political leaders who consistently back the fight — in part because the gay vote is so big — and a close-knit medical community in which many of the top officials in the fight against H.I.V. have studied together, trained each other and done research projects together through the University of California, San Francisco.

Because of the tech boom, City Hall’s budget grew from $5 billion to nearly $9 billion in 10 years. For patients not eligible for federal or state aid, the city has its own health insurance: Healthy San Francisco. And it has been able to counter every cut in federal AIDS funds.

“One of my first budget questions is always, ‘Are they cutting our Ryan White money again?’ ” Mayor Edwin M. Lee said in an interview, referring to the 1990 law that pays for AIDS drugs. “Since 2011, we’ve backfilled more than $20 million — with very little debate.”

Wealth also eases the burden in a harsher way: Soaring rents have driven out many poor residents. Two-thirds of San Francisco’s infected are white or Asian; nationally, 63 percent are black or Hispanic. Older, wealthier, openly gay white men are more likely to be insured and well-informed about risks than young black and Hispanic men, the epidemic’s fastest-growing subgroup.

But the city’s chief strength has been a willingness to go first, to rapidly adopt tactics that work in pilot projects or clinical trials, before they are endorsed by public health organizations like the C.D.C. and the W.H.O.

In 1983, the city opened the first dedicated AIDS ward. In 1987, it tested the first antiretroviral drug, AZT. In 1992, it began distributing clean needles to drug addicts.

In 2010, it adopted “test-and-treat” — antiretroviral drugs given as soon as a patient tested positive. In 2013, it started the preventive drug program, PrEP — pre-exposure prophylaxis with Truvada, a two-drug pill — at no charge to the uninsured. By some estimates, 15 percent of the city’s gay men are now taking Truvada.

Numerous recent studies have shown that people taking antiretroviral therapy every day not only live longer, but also have so little circulating virus that they are highly unlikely to infect others even through unprotected sex.

But the shift to immediate drug treatment was a struggle. Dr. Grant Colfax, who was then the city’s director of H.I.V. prevention, met resistance from doctors arguing that drug side effects were too harsh to begin immediately and that patients not yet motivated by brushes with death might take medicines carelessly and develop resistant strains.

Community groups were also angry. Many had lucrative city contracts to hand out condoms and advice, which the city rewrote to require they do tests and provide medical care.

“We were accused of ‘medicalizing H.I.V.,’ ” Dr. Colfax said. “Which I found ironic.” But the new-infection rate had not budged for 11 years. “We couldn’t keep doing the same old thing,” he said.

He persisted, and won.

To identify infected people, the city increased testing and created a program called Rapid, the one that helped Rafael: It guarantees a quick doctor’s visit, with cab fare to get there if necessary, and help finding insurance.

The Magnet Clinic, where Rafael was diagnosed, did 9,600 H.I.V. tests last year. It lies in the heart of the Castro, the city’s rainbow-flag-bedecked gay mecca, and resembles a cheery cellphone store: The waiting room has couches, flowers, disco music and photographs of the Sisters of Perpetual Indulgence, a campy drag troupe. A mobile of smiling penis toys dangled over one examination table.

“We didn’t want it to feel like a jail cell,” said the nursing director, Pierre-Cedric Crouch, wearing the clinic’s signature “No Blame/No Shame” T-shirt. “And we have no stigma. You can come in saying you just slept with 20 guys and don’t know what a condom is, and we don’t criticize you. We help you out.”

Magnet’s social workers will enroll patients in insurance or, if they have coverage, fight to make sure it covers what they need.

Jayne Gagliano, the benefits manager, said she regularly had to explain to out-of-state insurers that using Truvada to prevent infection was, in fact, F.D.A.-approved.

“The fragmented American insurance system is one of our biggest obstacles,” said Dr. Diane V. Havlir, the chief of the H.I.V. division at San Francisco General, who has seen patients stop taking their pills because of coverage lapses.

East of the Castro is the Tenderloin — a grim neighborhood peppered with single-room-occupancy hotels, homeless shelters and liquor stores. More than a quarter of the city’s homeless are gay or transgender, and many are found here. Some trade sex for drugs or a place to sleep, and end up infected.

Four years ago, San Francisco created Linkage into Care teams — Lincs for short — to track such patients.

In 2012-13, Lincs “navigators” searched for 315 missing H.I.V. patients and enrolled 116 of them — the rest were not found, or were jailed, dead or refused help. Navigators call and text patients, visit them and even escort them to doctors.

Some 73 percent of the Lincs patients stayed in care, and they were twice as likely to be virally suppressed as similar patients who were not in Lincs.

Recently, Erin Antunez, a Lincs navigator, spent her day focused on three: a young man released from jail that morning, a drug injector living in a Market Street hotel, and a 36-year-old mother staying in a former AIDS hospice.

The former prisoner “was not a big red flag,” she said. He usually took his pills and had a clinic appointment that afternoon.

The drug user often disappeared. He recently had texted her: “Where can I get an abscess drained without all the red tape?” but then had not answered several messages.

“A lot of this work is ‘Where’s Waldo?’ ” Ms. Antunez said.

Her first visit was to the Maitri Hospice, a haven on a residential block. Founded, according to its website, by a “drug-addled drag queen turned Zen master,” it has a Buddha and origami cranes for décor, and separate smoking porches for tobacco and medical marijuana.

The young mother was not dying, although she had survived two types of AIDS-related pneumonia.

Rather, she “just needed a break,” Ms. Antunez said, from methamphetamine binges in her housing project, during which she slept on any free couch and neglected her pills.

Today, she was waiting in a turquoise tank top, floral shorts and a white Tilley hat, ready to ride a city bus to Ward 86.

Ms. Antunez asked about her new tattoos. Deeply shy, the woman focused on her phone and mumbled childlike answers — until she mistakenly thought a reporter was a supervisor assessing Ms. Antunez, and leapt to her defense: “Erin is great! You can tell some people don’t want much to do with you, but she knocks on doors, she calls me, we play phone tag.”

Ms. Antunez’s afternoon client was not in his room at the Donnelly Hotel although she knocked loudly in case he was in a stupor.

A manager said he had “shown everyone his abscess” and then left.

Worried that he would lance his own abscess, she checked restricted databases that Lincs has access to, including all public clinic visitors and all H.I.V. tests ordered.

They revealed that her ex-prisoner and the drug user had seen doctors that afternoon. “Yeah!” she cried, waving her fists and doing a dance in her chair. “That’s a good navigation day.”

In 2013, shortly after the Food and Drug Administration approved the use of Truvada to prevent H.I.V. infection San Francisco started offering it to everyone at risk. Some gay men called others “Truvada whores” for avoiding condoms.

The backlash has mostly faded, said Dr. Bradley C. Hare, the director of H.I.V. for Kaiser, which also offered PrEP early. The first adopters, he said, “were the equivalent of the people who camp outside the Apple store for the latest iPhones.” Now a “second wave” has emerged, he said. “It’s people who needed a little more education but now say, ‘My friend is using it, and I’m doing the same things he’s doing, so ...’ ”

Dr. Lisa Capaldini, a popular doctor in the Castro, said she was “still ambivalent” about PrEP. Avoiding condoms, she said “makes it a Trojan horse for syphilis and gonorrhea, so if I had an 18-year-old gay son, I’d say, ‘I’d rather you used condoms.’ ”

Dr. Buchbinder, of the health department, disagreed, saying, “Denying PrEP to patients because they might have unsafe sex makes about as much sense as our colleagues who treat high cholesterol denying statins to theirs because they might eat more ice cream.”

A recent study of the first 657 Kaiser patients on PrEP found that their condom use had indeed gone down and more had caught venereal diseases — but in two years, not one had caught H.I.V.

Stephen, a slim, soft-spoken 24-year-old Chicago native, just got on Truvada. He delayed for a year, he said, until he found a job with health insurance. He had been on his parents’ policy and feared their seeing the prescription. They are conservative Roman Catholics who know he is gay but dislike it. “My father offered to buy me hookers,” he said, shaking his head.

Stephen first heard of the drug through a gay student group at his Catholic college. “But my friend said it means you’re super-slutty, that it’s for people who bareback, or go into dark rooms for sex,” he said. “I didn’t want to associate with it.”

Now, he said, he considers it another form of protection, “something I definitely want to add to my bag of goods.”

If he had grown up in San Francisco, he probably would have learned of it earlier. Discussion of PrEP is now mandatory in public school ninth-grade sex-ed classes.

The city’s success is not only shrinking the epidemic. It is changing the psychology of gay sex here in unexpected ways. The fear of death — so long a part of being gay in America — appears to be receding.

Take Bradley and Paul, a couple visiting the Magnet clinic for gonorrhea tests. Paul, 53, a ruddy and weather-beaten Hawaii resident who called himself “one of the dinosaurs,” said he had probably been infected with H.I.V. as a teenager. Many friends died decades ago. “It’s crazy I’m still alive,” he said.

Bradley, who resembled a prep-school athlete about 20 years younger, was from Atlanta and uninfected. They felt safe together, Paul explained, because he takes Complera, a triple-therapy pill, while Bradley takes Truvada.

“PrEP has really changed the game,” he said. “Bradley was the first guy I knew who was on it. He’d prefer to be with someone with an undetectable viral load than with someone who says, ‘Well, I assume I’m still negative.’ It blows my mind to be positive and not feel like a pariah.”

“It’s really nice to still be around now and see a younger generation go through what we did,” he added. “I grew up in the ’70s with no fear. The guys in the ’90s were full of fear. Now guys are growing up with no fear at all.”