Jimmy Carter’s five decades of leadership in global health brought a hideous disease to the brink of elimination, helped deliver basic health and sanitation to millions of people and set a new standard for how aid agencies should engage with the countries they assist.

It was quiet work and drew relatively little attention because it was focused on afflictions that plague the poorest people in the most marginalized places, but it had enormous impact.

“The work in global health may turn out to be some of the most important work that he did,” said Dr. William H. Foege, who helped lead the successful effort to eradicate smallpox in the 1970s and played a key role in drawing the former president into the field of global health after he left office.

Mr. Carter, the former president who died on Sunday at age 100, saw his health-care work through the prism of a larger effort for basic rights and as a tool for peace building.

“We believe access to health care is a human right, especially among poor people afflicted with disease who are forgotten, ignored and often without hope,” he wrote in 2001 after a trip to lobby Latin American leaders on neglected diseases. “Just to know that someone cares about them not only can ease their physical pain but also remove an element of alienation and anger that can lead to hatred and violence.”

He used his rare status as a former head of state to lobby presidents and prime ministers on behalf of their poorest citizens. He and his wife, Rosalynn Carter, trekked to remote corners of countries including Chad and Ethiopia to visit and comfort the sick. Then they traveled back to capital cities where in private meetings or, if those did not produce results, news conferences, they pressed for action on behalf of those same people.

Mr. Carter negotiated a cease-fire in Sudan’s decades-long civil war so that teams working to wipe out a parasite could reach civilians, and he brokered a major drug-donation program that has been key to progress in eliminating river blindness, another scourge.

The Carter Center, a nonprofit that Mr. and Mrs. Carter founded in 1982, is admired in countries where it provides development assistance as an organization whose beneficiaries set priorities and lead projects, an ethos that former staffers say originated with the Carters.

Dr. Foege was the director of the Centers for Disease Control and Prevention during the Carter presidency. Not long after Mr. Carter left office, he called Dr. Foege and asked him to run the Carter Center, through which he hoped to work on conflict resolution and advance democracy.

Dr. Foege said he told Mr. Carter that there were so few people working in global health at that time that he could not imagine leaving the field.

“A couple of days later, he called me and he said, ‘Would it make a difference if I became interested in global health?’” Dr. Foege said. “And that’s exactly what happened. And he became so interested in it. You could not give him enough material to study. He became better at global health than almost any global-health person I know.”

Mr. and Mrs. Carter decided to focus on neglected tropical diseases, which draw little investment in research or treatment because they infect the poorest people in the poorest countries.

“He would often ask, as we were trying to brief him on a new area that we were working on, ‘Who else is working on it?’” Dr. Foege recalled. “He said, ‘If there are other people working on it, you don’t need me.’”

The Carters learned about Guinea worm, an excruciating and disabling parasitic infection in which a three-foot-long worm slowly burrows out through the skin of the person it has infected, and for which there is no treatment other than painstakingly drawing out the worm over days or weeks. With Dr. Foege and his team, the Carters set the goal of eliminating the disease.

Mr. Carter did not, as he often vowed to do, live long enough to see eradication. But in 1986, when they began the campaign, there were an estimated 3.5 million cases in at least 21 countries. This year, as of early December, the Carter Center reported a provisional total of just 11 cases.

The progress has been achieved without the help of a treatment or vaccine, which was essential to eliminating smallpox; it has instead been the result of the slow and laborious work of helping people gain access to clean water to avoid infection by the waterborne offspring of an adult worm.

In 1995, Mr. Carter personally helped negotiate a cease-fire in the civil war that had then been being fought in the south of Sudan for more than a decade. The truce, which lasted nearly six months, allowed health workers to hunt for the worms and distribute water filters in areas that they had never been able to reach. The agreement also helped start a long process of negotiation between the government and rebel leaders that eventually led to the independence of South Sudan.

Mr. Carter also brokered deals with big pharmaceutical companies. In the 1980s, the drug company Merck & Company had begun a large-scale donation program of a drug called Mectizan to treat onchocerciasis, or river blindness, another parasitic disease transmitted by small flies in Africa, Latin America and the Middle East.

“We said we would give it to people for as long as was needed, but we needed to get it to the villages,” said Dr. P. Roy Vagelos, who began the program in 1987 as the chief executive at Merck.

The Carter Center agreed to incorporate the elimination of river blindness into its activities, delivering a Mectizan pill to everyone in infected areas — even the most isolated communities — once a year.

A few years later, Dr. Foege was at a meeting in Paris, where the World Health Organization reported that Mectizan combined with another drug, Albendazole, could treat lymphatic filariasis, a parasitic infection that can lead to the condition elephantiasis. He was called out of that meeting to take a call from Mr. Carter. It was 5 a.m. in Georgia, but the former president was calling from home to say he had met a pharmaceutical company executive the night before who was keen to be part of a drug-donation program like the one Merck ran for Mectizan.

That company, SmithKline Beecham, it turned out, made Albendazole, and Mr. Carter coordinated an arrangement to send Mectizan and Albendazole to patients in Africa and Latin America that has helped sharply reduce transmission of lymphatic filariasis in countries including Nigeria and Ethiopia.

In 2001, Bill Gates and Melinda French Gates consulted Mr. Carter when they were establishing priorities for the charity they had just set up with the world’s largest endowment. Mr. Gates said that the former president urged them to expand their work in global health.

Mr. Carter’s encouragement, Mr. Gates said, helped dispel skepticism he had about the public-health field.

“When you look at it and say, Why isn’t anybody working on these things?, then you think, Well, maybe it means that it’s just impossible because of the corruption or delivery,” Mr. Gates said. “Jimmy and Bill Foege said, No, it really can work. You can get this stuff delivered.”

Mr. Carter arranged key introductions to African leaders, Mr. Gates said. He noticed that while the Carter Center was working in many countries, it did not have its name displayed prominently on projects and instead foregrounded the work of local leaders.

Ms. French Gates said that early on she asked Mr. Carter what he had learned over time that might help the Gateses avoid the same mistakes. “He said, Melinda, anything you do in global health, anything, you have to make sure you have the community bought in and they see it as their work — not as your work, as their work. That way they will own it and they will create lasting change,” she recalled him saying.

“That was just invaluable to me,” she said.

At one point, the Gates Foundation was considering work in the development of new drugs and vaccines, but Mr. Carter urged a different priority: “He said, If I were you all, I would spend more money on the diseases that affect people today,” she said. That led to a balance in the foundation’s giving between funding for future vaccines and “relieving some pain and suffering today,” she said.

Dr. Tedros Adhanom Ghebreyesus, the director general of the W.H.O., first encountered Mr. Carter while serving as the health minister of Ethiopia. The Carter Center was working on neglected diseases in Ethiopia, but Dr. Tedros’s priority was ending child malaria deaths, and he was short three million bed nets in a plan to distribute 20 million. He told the staff of the Carter Center, which did not do malaria work in Africa (because that effort was already comparatively well funded) that malaria, not the rarer diseases, was his chief concern. When word got to Mr. Carter of Dr. Tedros’s need for more nets, he quickly responded that the center would source them.

“So that showed me what kind of person he is: empowering and trusting. He doesn’t come in and say ‘I know better than you.’ He says, ‘You know your problems and you know the solution.’ It comes from humility and humbleness. He doesn’t come to dictate, he comes to help you, with sincere help.”

Dr. Yao Sodahlon today runs the Mectizan-donation program for river blindness, which is administered by the Task Force for Global Health, an organization founded by Dr. Foege. He survived a case of river blindness himself as a teenager in Togo, long before Mectizan. The drug used to treat him then was painful and toxic, with terrible side effects and low odds of success. But he regained his sight and became a doctor.

In 2004, the Carters came to Togo because the Guinea-worm program was not getting the cooperation from the government that the Center needed. Dr. Sodahlon, then a young medical officer, was asked to emcee an event to welcome them.

“The president and the first lady came down to Lomé to see the high-level officials and, of course, when he reached anywhere, everybody’s shaking, I can tell you: That’s his power,” Dr. Sodahlon said. Members of government who had been botching Guinea-worm eradication suddenly had to explain to the country’s leadership why a former U.S. president was arriving in search of answers.

“That’s how he worked. His name alone made every leader, every high-level politician, in every country shake,” Dr. Sodahlon said. “He had two tools: grab his phone and call the leader. And if not enough, take a plane and go to the country.”

Carter Center beneficiaries receive anti-parasite drugs once a year, and that might constitute their entire interaction with the health system, Dr. Sodahlon said: “They are people who are voiceless. They are suffering in silence. They are not bothering any politician. They have no political power.”

Not to the Carters, he said.

“They had power,” he said, “and they knew how to use the power to alleviate suffering from those who needed it most across the world.”