In Boston, Farmer had befriended a retired construction magnate and Second World War veteran named Tom White, and he brought him to Haiti to see how people were living. White began donating money for such necessities as concrete floors and tin roofs, and ended up financing the construction of a clinic. “I was twenty at that point—I didn’t even have an undergraduate degree,” Dahl said. “Paul was a medical student. But we all sort of rolled up our sleeves and did whatever there was to do. We were planting trees on the hillside to try and stop the erosion. We were writing to people, long letters asking for money; driving to Port-au-Prince to pick up tons of things, like cough medicine and bar soap.” When the clinic opened, with three examining rooms staffed by Farmer and several Haitian doctors, patients poured in from all over the country—travelling on foot for days to reach them, sometimes carrying sick loved ones on their backs, sleeping outside while they waited their turn for care. “That’s how few the options were,” Dahl said. “It was tiny, but we left all this rebar sticking out the top because we knew that we would need to build more. The rebar sticking out felt like a beautiful thing, because it said, ‘We know this isn’t enough, not nearly—but it’s what we can do right now.’ ”

In Cange, Dahl realized that there were ways besides being a doctor to improve the health of the poor. This was reassuring, because she had found that she was interested in science only up to a point. “I loved thinking about life cycles, or mosquitoes and how they’re connected to these tropical diseases and how a parasite goes through the liver of a sheep before it’s recirculated,” she said. “I mean, that stuff—that’s literature. That’s the poetry of systems.” Literature, ultimately, is what Dahl decided to study, at Wellesley College. In Boston, she and Farmer got an apartment together. “We would talk and talk and talk about what it was we were doing,” Dahl said. They believed passionately that their aid work was about “redistributive justice,” as Farmer put it to Tracy Kidder, who wrote a biography of him. Many development professionals advocated a doctrine of self-reliance, typified by the slogan “African solutions to African problems”—which Dahl and Farmer felt ignored the West’s role in creating those problems. “In the affluent world, history gets erased,” he told me. “Erasing history is unfair to some people, and it’s fine for others. At med school, nobody even talked about these things.” He sighed. “I hope I wasn’t uppity and sanctimonious to my classmates, but I probably was.”

At Harvard, Farmer met Jim Yong Kim, a young man from a family of South Korean immigrants, who, like him, was pursuing a Ph.D. in anthropology along with a medical degree. With Dahl they formed a trio. “We all read liberation theology together, and Ophelia was sort of a keeper of the faith,” Kim, who is now the president of the World Bank, recalled. They were particularly taken with the Peruvian philosopher and priest Gustavo Gutiérrez’s conception of a “preferential option for the poor.” Because God favors the poor and the powerless, Gutiérrez argued, Christianity should focus on the injustices visited upon the destitute. To Dahl, Farmer, and Kim, it seemed clear that this doctrine applied to health care, too. “The thing about looking for some grand theory, like Marxism or whatever ‘ism,’ is that a lot of those sort of peter out or are eventually discredited,” Farmer said. “It was hard for me to see how you could discredit an injunction to serve poor people preferentially. You don’t have to be an epidemiologist to realize that infectious diseases make their own preferential option for the poor—they afflict them more, and worse.”

Dahl, Kim, and Farmer drew up a mission statement for what they called “the Project.” They would work toward providing health care that prioritized poor people’s needs, rather than the cheapest or the easiest intervention. “You don’t say, ‘When I’m in Boston, I have this one set of standards, and when I’m in Haiti or Rwanda I’m just going to lower the shit out of them,’ ” Dahl said. Tom White, who had by then decided to systematically divest himself of his wealth, donated a million dollars of seed money. “We were not going to be a regular self-congratulatory do-gooder organization,” Kim said. “We were going to grapple with deep questions of responsibility. Ophelia was the one who could explain to all the people coming around why our approach was different.” Dahl had a knack for disarming defensive people and for convincing donors that the world’s most fortunate people had a moral obligation to investigate—and compensate for—the suffering that underlies their comfort. “If we can’t connect our own good fortune with the misfortune of others, then we’ve missed the boat completely,” she often said.

But, soon after the organization came into being, Dahl and Farmer’s romance fell apart. In 1989, she sent a letter explaining why she would not marry him. “You pointed out to me once, during an emotional argument, that the qualities I love in you—that drew me to you—also cause me to resent you,” she wrote. “Namely your unswerving commitment to the poor, your limitless schedule and your massive compassion for others.” Farmer didn’t speak to Dahl for nine months. Then they ran into each other at a restaurant in Cambridge. “I looked up and I saw him,” Dahl recalled, “and there was this recognition—just knowing that you’ve always known each other, and that we understood why we had to be apart, but hoped not to be apart again.”

“I’m still working with all the people I met then who are alive—because they’re willing to be involved!” Farmer told me. “The qualities you most need to do this are solidarity and empathy. Those are rare. Ophelia was, and is, exquisitely sensitive to other people’s suffering—she gets physically anguished about it. And that’s a wonderful thing to have.”

In 2013, Partners in Health opened a two-hundred-thousand-square-foot, three-hundred-bed teaching hospital in Haiti—the largest solar-powered hospital in the world. It is in Mirebalais, where Farmer and Dahl first met.

The economist William Easterly divides development agencies into two philosophically distinct groups: planners and searchers. In his 2006 book “The White Man’s Burden: Why the West’s Efforts to Aid the Rest Have Done So Much Ill and So Little Good,” he writes that the planner “thinks of poverty as a technical engineering problem” that can be solved with enough money, intelligence, ingenuity, and data. Searchers, by contrast, “are just on the lookout for favorable opportunities to solve problems—any problem, no matter how big or small.” The searcher sees that Cange needs a water spigot, or that Kangama needs an ambulance, and starts working to help obtain it. She may hope that eventually the strategy—and the money—will emerge to provide the entire region or nation or continent with water and ambulances. But she doesn’t wait for it.

Especially in the early years, Dahl said, “we adopted this stance that we’re not the experts—you tell us how we can accompany you.” But, as the clinic in Cange was overwhelmed by demand, it became clear that it was not enough to conduct a neighborhood census: they needed to partner with governments, so that the work they did could be coördinated and sustainable. “If you believe health care is a right, then you have to work with governments, because they are the only group that can confer rights,” Dahl said. “Ninety-nine per cent of people, the first thing they ask is, ‘What about corruption?’ It’s an easy nugget to grab on to. ‘There’s nothing to be done. Let’s sleep now, free from worry.’ ” Though P.I.H. won’t collaborate with a government that is dictatorial or hopelessly dysfunctional, it expects to have to work around patronage systems. “It’s not that we’re crazy bleeding hearts that just choose not to see any of this stuff,” Dahl continued. “It’s just that we don’t let it be a showstopper. Imagine if we shut things down whenever there was corruption in the United States: ‘Sorry, now you can’t have any health care.’ ” This reasoning reminded her of another common accusation: that white, Western people doing aid work in poor countries are practicing a kind of latter-day colonialism. “It’s often used as a not very eloquent excuse to do nothing,” she said. “It brings out the worst in one. You feel like saying, ‘So what are you doing?’ ”