Just six months after her arrival to Africa, however, Koenen was raped.
She and her sister, who was visiting for the Christmas holiday, had gone to visit another volunteer living in the Sahara desert city of Agadez. Her attacker was an acquaintance of the volunteer.
"He followed me into the house and then he started trying to grab me and kiss me, he started trying to pull me into the bedroom," she recounted in an interview. "He held me down with one arm on my chest and one arm on my legs. And then he raped me. The whole time I just couldn't believe this was happening to me."
Koenen said when she reported the rape to the Peace Corps, the in-country physician did a medical exam, but nothing else was done.
"My in-country director never came to visit me, never spoke to me," she said.
She gave a statement to local officials, and her attacker was brought in for questioning and then immediately released. She was told that the man said she had wanted to have sex, and the Peace Corps' in-country officials believed him.
"I would have gotten better treatment as a tourist than as a volunteer," she said. "It's insane."
Koenen said her treatment only got worse once she flew back, alone and of her own volition, to the organization's headquarters in Washington, D.C. According to Koenen, the Peace Corps put her up in a hotel with five other returned female volunteers who had also been victims of sexual assault. When she went to the Peace Corps' Inspector General's office to discuss what had happened to her, she said the woman she spoke with blamed her for the rape.
"I walked into her office and the first thing she says to me is, 'I am so sick of you girls going out with men, drinking and dancing, and then when something happens, you call it rape,'" recalled Koenen. " I felt like someone had just kicked me in the stomach."
Koenen quit the Peace Corps and went home to New Jersey. Because of the assault, she set out in a new direction professionally, and began to study the psychological impact of trauma.
Koenen, who now has a doctorate in clinical psychology and specializes in trauma, says she has devoted her career to making sure others who experience trauma are not treated in the same harmful way she was.
"I feel like my experience with the Peace Corps in D.C. was worse than the actual rape," she said.
Koenen was giving her young son a bath the night ABC News aired the report on Peace Corps' treatment of six volunteer victims of rape and sexual assault. A friend texted her to turn on the program.
"To hear not just one woman but this group of women, and since then coming in contact with more women who have had the same experience with the Peace Corps," she said, "I felt like I really had no choice but to come forward."
She had told close friends and family about the assault, but had never spoken publicly about it prior to the "20/20" report. She has since been in touch with other victim volunteers since the report, including the women who spoke out on "20/20," and has become involved in First Response Action, a non-profit organization advocating for improvement of Peace Corps' training and response to sexual assault.