AIDS has killed 35 million people. It’s caused physical pain and mental anguish for many who live with it. It’s created a generation of African orphans. It’s drained untold trillions of dollars from national economies and people’s pockets.

There’s also one other way to describe the AIDS saga. It’s a success story.

AIDS tells the story of mankind’s powers of observation, the capacity of science to figure things out, the importance of citizen movements, the globalization of problem-solving, the intolerance of extreme inequality, the impulse for generosity, the ability of government to do good. It captures everything that has made the turn of the millennium a time of optimism as well as crisis.

“There is no other infectious disease in the last 100 years that has caused so much suffering and so much death and appeared so unexpectedly,” said Diane V. Havlir, an AIDS physician at the University of California at San Francisco and co-chair of this year’s International AIDS Conference. “Also no other disease where the benefits of the investment in science and response have been so great.”

AIDS conferences are the stop-action frames of that narrative.

The 19th International AIDS Conference opens Sunday in Washington.

It is in the United States for the first time in a generation. Its convening on American soil acknowledges the end of a long and controversial U.S. policy — a ban on known HIV-positive people from entering this country. As a result, 25,000 researchers, activists, clinicians, social scientists and journalists will be around town and underfoot this week.

If by some chance there were a Rip Van Winkle character among them and he asked what had happened in the 22 years since an international AIDS conference was held in the United States, the answer would be simple.

Just about everything.

In 1990, when the meeting was held in San Francisco, AIDS was an almost uniformly fatal disease. The public and much of the medical profession feared it and its victims, mostly white homosexual men and intravenous-drug users. The one AIDS drug worked poorly.

AIDS advocates waged angry and occasionally violent protests for more research and better drugs. Pharmaceutical companies and many scientists resented the meddling by “non-experts” (but eventually came to include them in decisions). Preventing infection required difficult changes in behavior. The biological workings of the infecting agent, human immunodeficiency virus (HIV), was mostly a mystery.

In 2012, HIV infection is a dangerous but treatable disease. Many people will live with it for decades and die of other ailments. It is less feared and stigmatized, although many sufferers still live at society’s margins. There are now two-dozen drugs to fight the virus. They are expensive but available to nearly everyone who needs them in wealthy countries and taken by more than 8 million people in poor ones.

A vaccine against AIDS remains elusive. But there are strategies afoot that may further quench the global epidemic, which peaked during two decades separating the 1990 conference and this one. And now there are phrases such as “AIDS-free generation” and “cure for AIDS” in the air.

The public’s understanding of AIDS has also progressed.

A new poll by The Washington Post and the Kaiser Family Foundation sketches a picture of the host country — a United States that understands the sea change in AIDS of the past two decades but is unaware of many details of the progress of recent years.

International AIDS conferences — held since 1985, first annually and now every other year — are cacophonous, confusing, crowded, interesting and exhausting events. For a week, experts give plenary lectures updating listeners about the biology, epidemiology and treatment of HIV infection. New research is presented in 15-minute lectures, in poster sessions held in cavernous halls, and in late-afternoon symposiums. Activists meet, train and exhort. Drug and device companies show their wares. The famous and the unknown speak.

Each conference distills the mood, the news and the symbolism of its moment in AIDS history. In memory they call up feelings of despair, surprise and solidarity.

Berlin, in 1993, was the nadir. Scientists and activists heard that treatment with AZT alone did not prolong life and that the AIDS epidemic was devastating Africa’s societies and economies.

Vancouver in 1996 brought evidence that “triple therapy” — three anti-retroviral drugs taken daily — could stave off death and restore many people to health. In 2000, the conference in Durban, South Africa — the first in an African country — drove home the inequity in treatment between rich and poor countries.

By the Barcelona conference two years later, it was clear that Africans and Haitians could successfully take the complicated regimen of pills — and benefit from it — just like Americans and Europeans. That unsurprising finding left no excuses for not bringing AIDS care to the developing world other than lack of will, effort and money.

Each conference is in search of a take-home message. This one’s has yet to emerge. It may be that places such as the District of Columbia (HIV prevalence of 2.7 percent) have lessons to learn from places such as Rwanda (HIV prevalence of 2.9 percent), whose response to the AIDS epidemic has been widely praised.

For the moment, the message of the conference here is simple: I’m back.

Greatest gain

Look for the words “amplification” and “implementation” to be a big part of talk this week.

They are boring, polysyllabic words. Not like “sex,” “drugs” and “death,” which were the key words of some AIDS conferences. But they are words that signal success.

The world today knows how HIV is transmitted, what can be done to prevent its spread and how to treat someone once it is diagnosed. The issue is no longer what to do but rather whom to do it for, where, how quickly and at what cost.

That’s where “amplification” and “implementation” come in. How much do we want to amplify our successes? What is the strategy for implementing our hard-earned knowledge? Those are the big questions.

The greatest scientific achievement in the past 22 years is combination anti-retroviral therapy (ART), with wide usage beginning in 1996. A patient takes three drugs that block one or more steps in HIV’s replication. That drives the virus to undetectable levels in the bloodstream and allows the immune system to restore itself to health.

Treatment is now a “done deal” in the eyes of John G. Bartlett, the 76-year-old former head of infectious diseases at Johns Hopkins Hospital and one of the first people to notice the appearance of rare infections in unlikely patients.

In May 1981 he saw a Baltimore woman, a heroin addict, who had Pneumocystis carinii pneumonia. A month later, physicians in Los Angeles published a report on a group of gay men with that same “opportunistic” infection seen most often in cancer patients with severely damaged immune systems. It was the first description of AIDS.

In 1984, Bartlett set up the world’s second AIDS clinic at Hopkins. He has helped write the federal government’s treatment guidelines ever since.

In a recent conversation, he conceded that there will be new and better HIV drugs to come. But with many patients achieving an undetectable “viral load” by taking one pill once a day, the therapy battle has been won.

“We have the [pharmaceutical] armamentarium that will take care of all the patients that come to us — with the proviso they take the pills,” he said. “People who start now should almost always succeed.”

There’s an added bonus. People with a virus that is “fully suppressed” rarely transmit it. In a study of African couples published last year, the risk of passing on the infection was cut 96 percent.

That finding, long suspected, has led to the battle cry of “treatment as prevention,” which also will be a big part of this conference’s conversations. Expect to hear that the multibillion-dollar effort to get HIV medicines to the developing world needs to be increased by billions more because, short of a vaccine, AIDS drugs are the best tool for stifling the epidemic.

For the American public, this news is just starting to sink in.

In the Washington Post and Kaiser Family Foundation poll of 1,524 adults done in mid-June, only 49 percent of people say HIV treatment improves the lives of those who receive it “and also helps prevent the spread of the disease to others.” Forty-four percent say HIV treatment benefits patients but has no effect on transmission.

The vast majority, however, know the central truth about AIDS in 2012.

Eighty-nine percent say they agree with the statement, “It is possible for people with HIV to lead healthy, productive lives.” In fact, nearly half — 46 percent — say HIV infection is a “manageable chronic disease, similar to diabetes or high blood pressure.”

While most patients and physicians would argue with that second statement, it is an extraordinary change in public perception from the HIV-as-death-sentence view that held sway the last time an AIDS conference was here.

Treating the world

From the start, AIDS has been a global disease — a pandemic. The world is also a flatter, more intimate place than it was in 1990. So most of this week’s conversation will be about AIDS far from American shores.

According to UNAIDS, the United Nations AIDS agency, there are 34.2 million people living with HIV around the world (a number that, as it happens, is close to the death toll of 35 million since the start of the epidemic). More than two-thirds of those people — 23.5 million — live in sub-Saharan Africa, and more than 1 in 10 (4.2 million) in India or Southeast Asia.

The effort to bring anti-retroviral therapy to people in those regions is the most important development since the emergence of the treatment strategy itself.

Today, more than 8 million people in low- and middle-income countries are taking the medicines. That’s 54 percent of those who should be getting it, according to clinical guidelines drawn up by the World Health Organization.

Coverage in sub-Saharan Africa is 56 percent. (That is the region targeted by the President’s Emergency Plan for AIDS Relief, or PEPFAR, the five-year, $15 billion program that George W. Bush announced to astonished listeners in his 2003 State of the Union speech.) Coverage is higher in Latin America (70 percent), lower in Asia (40 percent) and much lower in Eastern Europe and Central Asia (23 percent) and the Middle East and North Africa (13 percent).

Less than 10 years ago, there were only 400,000 people in the developing world taking anti-retroviral drugs. The 20-fold increase has occurred with astonishing speed. In Africa, the number of people on therapy jumped 20 percent — from 5.1 million to 6.2 million — in 2011 alone. The Obama administration announced last week that PEPFAR is now treating 4.5 million people, a half-million more than in December.

Prevention is also making headway.

In 2011, 2.5 million people acquired HIV — one-fifth fewer than a decade earlier. Last year, 57 percent of pregnant HIV-positive women got anti-retroviral drugs to prevent transmitting the virus to their babies — a strategy that produced 24 percent fewer newborn infections than just two years earlier.

In the past few years, 1.3 million African men have been circumcised, which studies show reduces the risk of becoming infected. In 2010 there were 131,000 health facilities offering HIV testing and counseling, up from 107, 000 just a year before. Changes in sexual behavior driven by educational campaigns and condom use have prevented hundreds of thousands of new cases.

News of these recent gains appears not to have reached most Americans. Nearly three-quarters in the Post-Kaiser poll say that most people in developing countries do not have access to HIV prevention services. Only 16 percent say that most do.

However, when asked more generally about progress against AIDS, Americans are more up to date. Asked if “the world is making progress,” 58 percent say yes, a lot more than in 2002, when 35 percent gave that answer.

Paying for progress

So who’s paying for this progress?

Countries of the developing world last year spent $8.6 billion on AIDS. Rich countries, philanthropies, the World Bank and U.N. agencies spent another $8.2 billion. Money from the U.S. government accounted for 48 percent of all international assistance. When only assistance from governments is considered, the U.S. share was even larger — 59 percent.

A new entity, the Global Fund to Fight AIDS, Tuberculosis and Malaria, has collected or been pledged $22.6 billion since its creation in 2002. It now provides AIDS treatment for 3.3 million people.

This picture is changing, too.

In the past five years, 81 countries increased the amount of money they spent on AIDS by more than 50 percent. Even some of the poorest, such as Haiti and Sierra Leone, more than doubled it. South Africa quadrupled it.

“I am seeing a new narrative,” UNAIDS head Michel Sidibe, who is originally from Mali, said last week. “We are entering a new era — an era of burden-sharing but also an era of ownership. I hope that will be one message this week.”

That is not going to muffle shouts that the United States is not spending enough money on AIDS, despite the fact that PEPFAR’s budget is $6.6 billion. Activists are particularly unhappy with the Obama administration’s plan to cut the program by 3.3 percent next year in the name of fiscal austerity. The president’s commitment “has been lukewarm at best,” said Tom Myers of the AIDS Healthcare Foundation, which helps care for people in 26 countries. “It may be better if he doesn’t attend the conference.”

In fact, the White House announced that the president’s schedule will preclude him from coming to the conference. However, he will send a video message and host a reception at the White House “for people living with HIV and to thank those dedicated men and women who have been working on the front lines of the fight,” a spokeswoman said.

In making their case for more money, advocates will argue that overseas AIDS spending is a bargain in unexpected ways.

A study published in May found that mortality from all causes — not just AIDS — was 20 percent lower in African countries getting PEPFAR money than in neighboring countries not getting it. The reason is not known, but one theory is that better training and organization, new clinics and equipment, and a reliable supply chain for pharmaceuticals is benefiting entire health systems. The flood of money, it appears, is lifting all boats.

The Clinton Health Access Initiative, part of former president Bill Clinton’s charitable foundation, announced last week that a year’s worth of HIV care in Africa is cheaper than everybody thought. In Ethiopia, Malawi, Rwanda and Zambia it averages $200 a person. The reason is not just lower prices for drugs but also savings gained by shifting care of routine patients to non-physician practitioners.

In the Post-Kaiser poll, 46 percent of Americans agree with the statement, “The United States is a global leader and has a responsibility to spend more money to help fight the HIV/AIDS epidemic in developing countries.” Thirty-two percent say the United States is spending “too little” to prevent and treat HIV in such countries. Both sentiments are virtually unchanged from a decade earlier.

Asked about President Obama and his administration, 44 percent of Americans say he is “not doing enough” in developing countries. They are harsher on other players. Fifty-nine percent say pharmaceutical companies are not doing enough, and 72 percent say that is true of developing countries’ governments, too.

Drugs for prevention

On the domestic front of the war on AIDS, a different debate about “amplification” and “implementation” is getting started. It involves prevention, not treatment. The question is how many people — and which ones — should use a new weapon called pre-exposure prophylaxis, or PrEP.

Several studies have shown that when HIV-negative people take anti-retroviral drugs they are less likely to acquire the virus if they have a sexual encounter with someone who is infected. The protection is far from absolute. But in people who take the medicines regularly, the strategy appears to reduce risk by at least 75 percent. This is true for homosexuals and heterosexuals.

This month, the Food and Drug Administration approved the use of Truvada, the brand name for a combination of emtricitabine and tenofovir, for this purpose. Is this the secret to ending the spread of the AIDS virus? Some think it will be a big help, others that it is a dangerous and expensive distraction.

It is certainly expensive. A study published in April estimated that if every uninfected gay man in the United States took PrEP for the next 20 years, about 250,000 infections would be prevented. The cost would be $480 billion, or about $216,000 for every year of life saved by preventing disease and premature death. That price tag is too big for even the sky’s-the-limit U.S. health-care system.

However, if only gay men with the riskiest behavior (five partners a year and some other characteristics) use PrEP, the cost comes down to $50,000 per year of life saved. That’s only a little more than the cost of having healthy people take statins to lower their risk of heart attacks.

The population in most need of better prevention is young “men who have sex with men.” (Abbreviated MSM, it is the epidemiologically correct name for male homosexuals.) Infections among black people in this group are growing at an alarming rate — up 48 percent between 2006 and 2009.

An important insight in the past few years is that sky-high infection rates are not just a consequence of risky behavior. They can also reflect the fact that some populations have a lot more HIV in them. They are simply a riskier pool of partners.

That appears to be the case with the “sexual networks” of young African American homosexuals. They do not have high-risk sexual encounters (such as anal sex with a stranger and without a condom) more frequently than their white counterparts. Nevertheless, they are much more likely to become infected.

“It’s like Russian roulette. In some communities there may unfortunately be three or four bullets in the chambers,” said Gregorio A. Millett, an epidemiologist from the Centers for Disease Control and Prevention now working in the White House.

How much interest there is in PrEP is unknown. That is likely to be a topic of discussion at the conference this week. For some people, it is already a subject of internal debate.

A man who works at the Commerce Department said last week that he has been taking Truvada for four months to lower his risk of infection. He usually uses a condom. But once or twice a month, for various reasons, he has unprotected anal intercourse.

He is not doing that more frequently now that he is on PrEP, he said. But he did not deny that over time he might become more tolerant of risky encounters knowing that anti-retroviral drugs are streaming through his blood. Plus, he knows that HIV infection no longer spells death and that at age 57 he no longer has his whole life ahead of him. Both may become reasons for him to let his guard down.

“Let’s just say I feel less guilty about it — the times when I don’t use condoms and should have,” said the man, who spoke on the condition of anonymity to talk about his sex life. Of the new PrEP era, he added: “It’s really about weighing risks. This is sort of uncharted territory for me.”

Perhaps not surprisingly, this recent development has not yet sunk into American consciousness. Asked “Can the drugs used to treat HIV also be used by people who are HIV-negative to lower their risk of getting HIV?,” only 17 percent answered yes. Fifty percent answered no.

Kristina Meacham contributed to this report.