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More than 120,000 people have fled fighting in South Sudan’s Jonglei state. They are hiding in malaria-infested swamps without access to safe drinking water, food or medical care, the international medical humanitarian organization Doctors Without Borders said Friday.

Mr. Mading’s trip to South Sudan was made possible by the U.N. refugee agency.

“One family torn apart by war is too many” is the U.N. theme for World Refugee Day, which is commemorated each June 20.

Guor’s story epitomizes that theme,” said Charity Tooze, director of celebrity relations and special projects at the U.N. refugee agency office in Washington, who first reached out to Mr. Mading last year.

The refugee crisis created by the war in Syria has eclipsed other crises and diverted international attention.

Ms. Tooze described the situation in South Sudan as unique.

“It is a brand-new country that is trying to create stability with all these refugees on the border in what is the toughest part of Africa to get to,” she said.

Mr. Mading developed a passion for running while in high school in New Hampshire. He studied chemistry at Iowa State University and now lives in Flagstaff, Ariz.

Mr. Mading, who was not a U.S. citizen at the time, participated in the marathon at the Olympic Games in London last year under the Olympic flag, unaffiliated with any nation. He became an American citizen in February.

When he left Sudan, Mr. Mading swore he would never run again because it was something he associated with fleeing for his life.

“I didn’t know running is something you do as a sport,” he said. “But running has given me an opportunity to give back what I owe to people.”

Mr. Mading wants to help develop a sports program in South Sudan and use his experience to train young athletes.

As for his own goals, he hopes to run again in the Olympics in Rio de Janeiro in three years.

“Hopefully, in 2016 I will either represent South Sudan or the U.S.,” he said. “I see them both as my country.”