The purpose of this study is to compare the effects, good and/or bad, of posaconazole and micafungin in preventing fungal infections after chemotherapy for acute leukemia or myelodysplastic syndrome. When people take chemotherapy, they are more likely to get infections. Posaconazole has been approved for the prevention of fungal infections in patients who receive induction chemotherapy for acute leukemia and myelodysplastic syndrome. Posaconazole is available only as an oral suspension and has to be given with food. After chemotherapy, many patients are not able to tolerate food or oral medication because of severe mucositis. Patients unable to tolerate food and oral medications cannot take posaconazole.