"Good Morning America" co-host Robin Roberts, who five years ago beat breast cancer, said Monday that she has now been diagnosed with myelodysplastic syndrome, a blood disorder caused by chemotherapy for her cancer. She is now taking chemotherapy in preparation for receiving a bone marrow transplant from her sister later this year. Because she is relatively young and healthy, the combination of treatments should cure the condition, doctors have told her.

Myelodysplastic syndrome is sometimes known as pre-leukemia, and many researchers now believe that, if untreated, it will progress to acute myeloid leukemia. It most commonly strikes people between the ages of 58 and 75, but can occur at any age, particularly if the patient has had cancer chemotherapy. It is estimated to affect as many as 50 Americans per 100,000, with about 20,000 new cases each year.

It is a disease of the bone marrow -- the semi-liquid tissue inside bones that produces blood cells. Stem cells in the bone marrow develop into two types of cells, myeloid and lymphoid. Lymphoid cells go on to become white blood cells that fight infections. Myeloid cells develop into three different types of cells: red blood cells, which carry oxygen; platelets, which control bleeding by forming clots; and white blood cells. In myeloplastic syndrome the myeloid cells stop developing; they do not function normally and either die in the bone marrow or soon after they enter the blood. The dysfunctional cells crowd out healthy cells.

Symptoms are often not apparent, but can include shortness of breath, weakness or tiredness, pale skin, easy bruising and bleeding, and fever or frequent infections. The best treatment for the type of disorder Roberts is suffering is to kill all the stem cells with chemotherapy, then replace them with functioning stem cells from a donor -- in this case, her sister. Treatment is usually more effective when the disorder has been caused by chemotherapy.

Roberts announced her condition on the show and on the ABC blog, saying she will continue her job at "Good Morning America" and that "My doctors tell me Im going to beat this and I know its true."

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'Good Morning America' co-host Robin Roberts has blood disorder