When Ron Oppedisano received his cancer diagnosis in the winter of 2010, it was shattering.

"Your life stops, literally comes to a standstill," Oppedisano, then the mayor of Norridge, Ill., said, of the diagnosis of acute myeloid leukemia.

His daughter, Tena McCullough, agrees.

"It was one of the worst days that we've had as a family," she said.

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After several grueling rounds of chemotherapy, Oppedisano went into remission. Ron's wife, Linda Oppedisano, said the family thought he was cancer-free after that. He wasn't.

Two years later he had to put his re-election plans on hold when he had a recurrence. Without a bone marrow transplant, Oppedisano wouldn't survive for six months and, with no siblings, the Be the Match donor registry was his only hope.

Waiting to see if someone would step up was tough, he said, and when Samantha Nielsen did, Oppedisano's entire family was grateful.

Nielsen joined the Be the Match donor registry by giving a swab of cheek cells. After further testing, Nielsen was deemed to be a "perfect match for Ron," according to Oppedisano's doctor, Patrick Stiff.

To prepare for the transplant, Oppedisano underwent what Be the Match calls a "conditioning regimen" of chemotherapy and possibly radiation therapy to prepare his body for the new cells. Donors donate their cells through either peripheral blood stem cell (PBSC) donation - a nonsurgical procedure that takes place at a blood center or clinic - or marrow donation, a surgical outpatient procedure that takes place at a hospital.

The transplant, in which Oppedisano received the new cells intravenously, was a success. On Dec. 4, 2013, Oppedisano - though technically 58 years old - celebrated his new "first" birthday.

"No amount of 'thank yous' could ever adequately express what I owe," Oppedisano said. "Every day I wake up, I thank God that I'm here."

Click HERE to find out more about Be the Match and how you can help.

Credit: ABC

His son, Vince, said it was because of Nielson that his father was still with the family.

Oppedisano's other daughter, Lisa Oppedisano-Gannon, shared her sibling's sentiment.

"He's such a great grandfather and to see him every time with Lia, it's just amazing ,because I know it was a possibility that he would never get to meet her," she said.

Oppedisano and Nielsen were recently allowed to exchange contact information, and "GMA" was there for their reunion.

"Hi Sam," Oppesidano said when he saw Nielsen, who lives just outside of Houston. "So good to see you and reach out and touch you and know you're real."

"You are the real hero, you really are," he added. "I wouldn't be here without you, I mean that, you've done so much for our family and for me, especially."

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Nielsen thanked him, then added: "I don't feel like a hero, I just feel like I made a small sacrifice."