NEW YORK — Five years after being treated for breast cancer, “Good Morning America” co-host Robin Roberts has a new health fight on her hands.

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, an anchor for WWL-TV in New Orleans, is regarded 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 in her sister, Besser said things look promising for her.

Roberts has contributed to “Good Morning America” since 1995, and was named co-anchor in 2005. The former Southeastern Louisiana basketball star worked at ESPN for 15 years.

She had blood tests that disclosed the MDS after feeling fatigued, or more fatigued than even someone who had to get up for a 7 a.m. show every weekday might expect, Besser said.

She learned of her diagnosis on the same day that “Good Morning America” beat “Today” for a week in the ratings for the first time in more than 16 years, Roberts said. On a day some of her bone marrow was extracted for testing, 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.

Roberts will take some occasional days off from “Good Morning America” depending on her reaction to treatment. She will probably need to take a couple of months away from the show immediately after the bone marrow transplants. Her current team of colleagues George Stephanopoulos, Josh Elliott, Lara Spencer and Sam Champion will pick up the slack for her, with occasional co-hosts like Elizabeth Vargas joining.

“GMA” has been on a ratings surge in recent months, more competitive with the “Today” show than it has been in years.