Streptococci are bacteria that live in one's mouth and gut. These bacteria can escape into the blood when the lining of the mouth and gut weakens from cancer therapy. This can make the person who is undergoing a bone marrow transplant very sick. All patients who get this infection are treated with antibiotics. Vancomycin is one drug that is used to treat this bloodstream infection once it is diagnosed. Studies have shown that giving vancomycin before a bone marrow transplant seems to prevent this infection. However, giving vancomycin too soon may increase the chance that the kidneys will be irritated. It may also increase the chance that other bacteria will become resistant to this drug. We, the investigators at Memorial Sloan-Kettering Cancer Center, do not know if waiting to start vancomycin until the patient has a first fever can also prevent this infection.