William Campbell’s eureka moment came in 1975, as he was testing a new medicine to fight parasites in domestic and farm animals. The Irish-born scientist realized a parasitic worm in horses bore an amazing resemblance to the parasite that causes river blindness in humans.

It was a simple connection, almost too good to be true, so he allowed himself only a moment’s excitement. “We’d see other [drugs] rise and fall,” Campbell, now 85, recalled in an interview Monday. “The probabilities are overwhelming that the whole thing is going to fall apart. Most drug candidates don’t ever reach the marketplace.”

In the end, a form of the drug that Campbell and others developed succeeded where previous treatments had failed. Their work resulted in a sharp decline in river blindness, a parasitic infection that has blinded tens of millions of people in Africa, Latin America and other poverty-stricken countries. It also helped reduce the incidence of filariasis, another parasitic disease that can result in elephantiasis, a painful and disfiguring swelling in the legs and lower body.

On Monday, Campbell and a fellow scientist who helped develop the treatment, Satoshi Omura of Japan, were awarded the Nobel Prize in medicine. Chinese researcher Youyou Tu, who discovered a drug known as artemisinin that significantly cut death rates from malaria, shared in the prize. The scientists also will share an award of nearly $1 million.

“These two discoveries have provided humankind with powerful new means to combat these debilitating diseases that affect hundreds of millions of people annually,” the Nobel committee noted in a statement. “The consequences in terms of improved human health and reduced suffering are immeasurable.”

Some 3.4 billion people in 100 nations are at risk of the diseases. Committee member Hans Forssberg said the trio’s work “has promoted well-being and prosperity for both individuals and society.”

Tu, 84, inspired by a description in a 1,700-year-old Chinese text of the use of sweet wormwood to combat fever, discovered artemisinin, which has been used by millions against malaria. She is affiliated with the China Academy of Traditional Chinese Medicine in Beijing.

The prize caps off a remarkable journey for Tu, a researcher who nearly 50 years ago began working on a classified Communist military project using clues from ancient Chinese medicine. She did much of her seminal work in secret, and for years her discovery of a cutting-edge drug developed from an ancient Chinese folk remedy was hardly known beyond the country’s borders.

The development of qinghaosu — or artemisinin — is now one of modern China’s proudest accomplishments.

Omura, 80, of Kitasato University in Tokyo, collected thousands of soil samples to isolate ones that contained promising antibacterial agents. Campbell, an expert in parasite biology who spent decades working for pharmaceutical giant Merck, picked up on Omura’s work and developed a compound known as avermectin, which proved in­cred­ibly effective at killing off parasites in some animals.

A modified form of that compound, known as ivermectin, showed similar results in humans. If taken annually, the drug relieves the agonizing itching that often accompanies river blindness and effectively halts the progression of the debilitating disease.

The Nobel Prize for physiology or medicine was jointly awarded to William Campbell and Satoshi Ōmura for their work on infections caused by parasites, and to Youyou Tu for her work on malaria. (Twitter/Nobel Prize)

In 1987, Merck pledged to provide the drug to anyone who needed it, free of charge, for as long as necessary. It established a donation program that works in partnership with the World Health Organization, governmental-health ministries and nongovernment health groups. Hundreds of millions of doses are distributed each year, many of them in remote parts of Africa.

Campbell, who spent many years as an associate fellow at Drew University in New Jersey after retiring from Merck, said Monday that the Nobel news came as “a tremendous shock.” He said he felt honored to represent a much larger group of researchers, including Omura, whose work led to the breakthrough.

But even as his phone kept ringing and neighbors kept knocking on his door — “I’ve never known a Nobel Prize winner!” one shouted in the background Monday morning — Campbell insisted his biggest reward came decades ago.

He recalled a visit to the small West African country of Togo to observe clinical trials for the drug he helped develop. There, he saw young boys whose lives would turn out very different partly because of his work.

“To see a blind man surrounded by little kids who are not going to be exposed to that risk, in a village where half the adult males were blind. . . . That’s obviously a very moving experience,” Campbell said.

He keeps a snapshot of a handful of those boys, most of them around age 12, in an album at his Massachusetts home. He also includes the image in most of his PowerPoint presentations.

“I look at that picture, and I think they must be healthy, productive adults now,” Campbell said. “That’s with me all the time.”

Jeff Guo contributed to this report.