Myelodysplastic syndrome is a disorder where stem cells in the bone marrow do not mature normally, resulting in reduced numbers of one or more types of blood cells. It most commonly arises in two settings - idiopathic MDS in those over 50 years old, or therapy-related MDS in patients who received previous chemotherapy or radiation treatment. Common symptoms include fatigue and increased risk of infection due to low blood cell counts. Prognosis depends on the percentage of immature blood cells in the bone marrow, with higher percentages associated with greater risk of progression to acute myeloid leukemia and shorter survival times.