A small number of patients will develop severe infection after chemotherapy or after allogeneic bone marrow transplantation. The ability of patients with leukemia or bone marrow transplantation to fight infections is decreased. One reason is that chemotherapy transiently destroys the type of white blood cells called neutrophils. Neutrophils are the first line of defense of our body against infections. Bone marrow transplant patients may be receiving edications to prevent graft versus host disease. Such medications may decrease the ability of the body to fight infections.