Micafungin is an antifungal medication that is given only intravenously. Micafungin is approved for the treatment of certain fungal infections and for preventing fungal infections in patients who receive bone marrow transplant. The investigators know that micafungin is safe. Micafungin has not been tested for the prevention of fungal infections in patients receiving chemotherapy for acute leukemia and myelodysplastic syndrome. Because micafungin is given by vein, it can be given even in patients who cannot take food or medications by mouth after chemotherapy. In this study the investigators want to compare micafungin to posaconazole when given for the prevention of fungal infections in leukemia and myelodysplastic syndrome patients.