Eva Kor poses at the CANDLES Holocaust Museum in Terre Haute, Ind., in 1999. (Bob Poynter/The Tribune-Star/AP)

Eva Kor was 10 years old when she arrived at Auschwitz, the Nazi death camp in Poland where 1.1 million people, nearly all of them Jews, perished in the Holocaust. On the selection platform, her mother held on tightly to Eva and her twin sister, Miriam, as their father and two older sisters disappeared into the masses.

“Twins?” an SS guard called out. “Twins?”

“Is that good,” Mrs. Kor remembered her mother inquiring, if her daughters are twins? The guard said yes, and the sisters were taken away. They would never see their mother or the rest of their family again.

Eva and Miriam were among 1,500 sets of twins subjected to medical experiments by the infamous Nazi doctor Josef Mengele. Fewer than 200 of those victims are thought to have lived.

Decades later, after making a new life for herself in Terre Haute, Ind., Mrs. Kor helped draw international attention to the suffering that the victims of the medical experiments had endured, as she preached a philosophy of forgiveness that she said unburdened her of her anger toward her persecutors.

Mrs. Kor died July 4 at a hospital in Krakow, Poland, about an hour’s drive east of Auschwitz, where she was traveling with the CANDLES Holocaust Museum and Education Center, which she founded in Terre Haute in 1995. She had visited Auschwitz roughly 30 times in recent decades with students, teachers, the news media and other groups, according to her son, Alex Kor, who said the probable cause of death was a heart or lung ailment. She was 85.

Mrs. Kor was one of the more visible members of the ever-dwindling group of survivors delivering firsthand testimony of the atrocities of the Holocaust. She was the subject of several documentaries, including “Forgiving Dr. Mengele” (2006) and “Eva: A-7063” (2018). She drew admiration — as well as criticism from some fellow survivors — for her public expressions of forgiveness of the Nazis.

“For the life of me I will never understand why anger is preferable to a goodwill gesture,” she once said. “Nothing good ever comes from anger.”

She was born Eva Mozes on Jan. 31, 1934, in Portz, a Romanian community that briefly became part of Hungary during World War II. She and her religiously observant family of farmers were the only Jews in town. Speaking to the National Catholic Reporter in 1999, she recalled math problems at school that asked, “If you have five Jews and kill three, how many are left?”

In 1944, her family was placed in a ghetto and then deported to Auschwitz.

The medical experiments conducted there and at other Nazi camps had three purposes, according to the U.S. Holocaust Memorial Museum: to improve medical treatment for German troops, to test new medical procedures and drugs, and to confirm Nazi views of the supremacy of the Aryan race.

Victims were subjected to extreme altitudes and temperatures, injected with pathogens and sterilized. They endured bone-grafting procedures and injections in their eyes to change their eye color. Many victims were permanently disfigured, sickened or weakened. Many more died.

“Everything in the world was done to me that would have killed me,” Mrs. Kor said years later in an interview, “and here I am alive.”

She recalled being stripped of her clothes and tied down by her arms as she endured repeated examinations lasting as long as eight hours. From one arm, her tormentors took blood. “They wanted to know how much blood a person can lose and still live,” Mrs. Kor said. In the other arm, she received injections, sometimes five at a time.

Once, she said, the experiments brought on a dangerously high fever.

“I was trembling,” Mrs. Kor told ABC News in 1999. “My arms and my legs were swollen, huge size,” with red patches. Mengele examined her, she said, and pronounced that she had two weeks to live.

Her sister, Miriam, sustained kidney damage so severe that her kidneys stopped growing; in 1987, Mrs. Kor donated a kidney to her.

After their liberation from Auschwitz on Jan. 27, 1945, the sisters stayed in an orphanage before making their way back to Romania, where they lived with an aunt. In 1950, they migrated to Israel, where Mrs. Kor lived on a kibbutz and served in the Israeli army before marrying Michael Kor, a Latvian-born Holocaust survivor, in 1960.

Later that year, she settled with her husband in Terre Haute. He survives, as do their two children, Alex Kor of Carmel, Ind., and Rina Kor, of Terre Haute. Her sister, Miriam, died in 1993.

For years, Mrs. Kor said, she fought feelings of anger and bitterness.

“If childhood is the foundation of a human being’s life,” she once told the New York Times, “then what kind of foundation can we have if our memories are filled with the smell of burning flesh and chimneys belching the black smoke of our families?”

After the miniseries “Holocaust” aired on television in 1978, inviting some Americans to reckon for the first time with a history they might have preferred not to confront, Mrs. Kor began speaking publicly about her experience in the camps.

In 1984, she and her sister founded CANDLES (Children of Auschwitz Nazi Deadly Lab Experiments Survivors) in an effort to locate other survivors of the medical experiments. To date, according to the organization, they have found 122 individual Mengele twins in 10 countries.

Mrs. Kor returned to Auschwitz for the first time in 1985. A decade later, on the 50th anniversary of the liberation, she visited the camp with Hans Münch, a former Nazi doctor who agreed to offer a testimony of what had transpired at the camp. In an emotional ceremony, Mrs. Kor announced that she forgave the Nazis.

“It was a life-changing experience,” she later said. “I realized I had power over my life. I had the power to heal the pain imposed on me in Auschwitz by forgiving the people who imposed that pain.”

Critics argued that it was not Mrs. Kor’s place to offer such a pardon and that perhaps none was even possible. She emphasized that she spoke only for herself and that forgiveness was for her benefit, not for that of the Nazis.

In 2015, Mrs. Kor appeared in Germany at the trial of 93-year-old Oskar Groening, a Nazi official dubbed the “bookkeeper of Auschwitz” who was charged with 300,000 counts of accessory to murder for his part in the operation of the camp. Even as she testified against him, she called for him not to be imprisoned, but rather for him to speak to youths about the tragedy of Nazism. They hugged, and she received a kiss from him on the cheek.

Other survivors who acted as plaintiffs in the case angrily distanced themselves from Mrs. Kor’s actions, saying in a statement that “we cannot forgive Mr. Groening his participation in the murder of our relatives and another 299,000 people — especially since he feels free from any legal guilt.”

Groening was convicted and sentenced to four years in prison. He died in 2018 without serving any of his sentence.

In the United States, Mrs. Kor did drafting work before becoming a real estate agent. She received a bachelor’s degree from Indiana State University in 1990.

Her museum in Terre Haute was destroyed by arson in 2003 but reopened in 2005 after receiving donations of almost $300,000 — a response, Mrs. Kor noted, that brought even greater attention to her cause.

“What I tell everybody is that you — any victim, any person hurt — you have the same power,” she told the Indianapolis Star. “You have the power to forgive. And what it does, forgiveness, has nothing to do with the perpetrator. It has everything to do with the way the victim feels.”