Good Morning America co-anchor Robin Roberts announced Monday morning that she’s been diagnosed with a condition called myelodysplastic syndrome, or MDS.

MDS is a rare blood disorder in which “the bone marrow produces enough blood cells, but they’re “fragile,” or “cracked,” so when they try to get into the blood stream to do what they do, they break apart prematurely,” explains Martin Tallman, chief of the leukemia service at Memorial Sloan-Kettering Cancer Center in New York.

“If they’re not circulating in sufficient quantities,” Tallman says, the white cells can’t properly fight infection, the red cells can’t carry oxygen and the platelets can’t help blood clots form to halt bleeding. Tallman notes that the condition used to be called “pre-leukemia,” but that term no longer is commonly used because “not everyone who has it will develop leukemia.”

Tallman says the condition can linger for years in some patients without needing treatment, while other patients will need therapy right away. Roberts reported Monday morning that she was preparing for a bone-marrow transplant, with her sister serving as the donor.

Tallman says the medical community has “shifted away” from calling such procedures “bone marrow transplants,” which term he says has largely been replaced with “stem cell transplant.” In that procedure, he explains, “the patient receives chemotherapy and sometimes radiation to kill the bad cells.” When the healthy blood cells are instilled, Tallman explains, “Stem cells grow like seeds in a garden and reestablish normal blood cell production.”

Patients receiving such transplants are at increased risk of infection and can also experience “graft versus host disease” in which the immune cells that inevitably are transplanted into the patient along with the healthy stem cells actually reject the patient, Tallman explains. That situation “can be life-threatening or debilitating” but usually can be treated, he says.

Even with its risks, Tallman explains, stem-cell transplant is the only known cure for MDS. The FDA has approved three drugs to treat the condition, but none of those actually cure it, he says.

Tallman was unable to comment directly on Roberts’s case or her prognosis. But he says, “It is true that if you develop MDS subsequent to chemotherapy, you tend to have unfavorable genetic changes” to your cells that suggest a less-favorable prognosis. Roberts was treated for breast cancer five years ago; the treatment reportedly included chemotherapy.

Though people of all ages can develop MDS, risk increases with age. Tallman believes the condition, which affects an estimated 15,000 to 20,000 people per year in the United States, is more common than believed. “I think it is under-diagnosed, particularly in older adults” in whom the condition’s presence may be masked by symptoms of other illnesses and conditions.