Robin Roberts, morning anchor on ABC's "Good Morning America," is off duty this week because of opportunistic infections months after a bone marrow transplant for her myelodysplastic syndrome.

After feeling "under the weather" during a Key West vacation last week, she contacted her doctors and returned to New York City, she announced in a Facebook update this morning, and seems to be doing just fine after several days in the hospital.

The morning show host underwent the bone marrow transplant to treat myelodysplastic syndrome, a rare blood disease in which the bone marrow doesn't make enough healthy blood cells. The disorder leads to anemia, shortness of breath, easy bleeding, and increased risk of infection, and can turn into a type of leukemia if left unchecked.

The cause of myelodysplastic syndrome is unknown, though according to the National Cancer Institute, age and exposure to certain chemicals can increase a person's risk of developing it.

After revealing her diagnosis last summer, Roberts took a six-month medical leave to prepare for a bone marrow transplant in September. The tissue was donated by her sister, and Roberts went through intensive chemotherapy. She returned to "Good Morning America" two months ago, and was in good health until this recent bout of illness.

"Seems my young immune system needed a little boost to fight off "opportunistic infections," she explained on Facebook.

"My doctors assured me that this was NOT because I was working or doing too much, too soon. It's extremely common, post bone marrow transplant, to have complications. I'm blessed that mine have not been severe."

Roberts reassured fans that she is "feeling MUCH better, and will relax at home for the rest of the week."

She expects to return to "Good Morning America" next week, and included a heartfelt message expressing remorse that she was unable to cover the aftermath of the Boston Marathon bombing due to her illness:

"My heart continues to go out to the remarkably strong people of Boston. We stand by you now and always."