Roberts said Monday she was beginning chemotherapy treatment for myelodysplastic syndrome, or MDS, a blood and bone marrow disease once known as preleukemia. She is expected to get a bone marrow transplant sometime this fall.

Her older sister, Sally Ann Roberts, is seen as a perfect match to donate marrow and will be doing so.

“My doctors tell me I’m going to beat this, and I know it’s true,” Roberts, 51, said on the show Monday.

Roberts developed MDS as a result of her breast cancer treatment — a manner of transmission so unusual it affects only a few hundred people per year, said Dr. Richard Besser, ABC’s medical correspondent.

The prognosis for many MDS patients is dire, but that’s largely due to the disease primarily affecting people over age 60, Besser said. Between Roberts being young and healthy, and having already located a good donor, Besser said things look promising for her.

On a day some of her bone marrow was extracted for testing earlier this year, Roberts learned she had landed an interview with President Barack Obama where the president revealed his support for gay marriage.

“The combination of landing the biggest interview of my career and having a drill in my back reminds me that God only gives us what we can handle, and that it helps to have a good sense of humor when we run smack into the adversity of life,” she said.

First lady Michelle Obama, via Twitter, told Roberts that “Barack and I have you in our prayers. We believe in you and thank you for bringing awareness and hope to others.”

Roberts hopes that attention paid to her diagnosis will encourage people to donate bone marrow that might help someone else with the disease. For donors, it has become a relatively simple procedure comparable to donating blood, Besser said.