Robin Roberts, who beat breast cancer five years ago, is again battling a serious health condition.

The "Good Morning America" anchor announced on the air Monday that she is suffering from myelodysplastic syndrome (MDS), a blood and bone marrow disease.

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Surrounded by her colleagues during the announcement, Roberts appeared confident that she will conquer the disease, noting that her sister is going to be her donor for her bone-marrow transplant.

"My big sister is a virtually perfect match for me ... and  she is going to be my donor. The doctors tell me that is going to be a tremendous help in me beating this," Roberts said.

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"Bottom line, I am going to beat this," Roberts added. "My doctors say it, and my faith says it to me ... It's about focusing on the fight and not the fright."

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Roberts, 51, suggested that the disease might have resulted from her breast-cancer treatment. The anchor said she will begin pre-treatment on Monday, and will undergo the transplant either this summer or in the fall.

According to Roberts, she learned that she has the disease some time ago, at the same time that she was experiencing a professional high point.

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"I received my MDS diagnosis on the very day that 'Good Morning America' finally beat the 'Today Show' for the first time in 16 years," Roberts said in an announcement to her fans. "Talk about your highs and lows! Then a few weeks ago, during a rather unpleasant procedure to extract bone marrow for testing, I received word that I would interview President Obama the next day. 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 absurdity of life."

During Monday's broadcast, Roberts added that she will remain on the air, but will "be out for a chunk of time" when she undergoes the procedure.