Stem cell transplant reduces a person's ability to fight infections. There is an increased risk of getting new viral infections or reactivation of viral infections that the patient has had in the past, such as cytomegalovirus (CMV), Epstein-Barr virus (EBV) and adenovirus (ADV). There are anti-viral medicines available to treat CMV, EBV and ADV infections, though not all patients will respond to the standard treatments. Moreover, treatment of viral infections is expensive and time consuming, with families often administering prolonged treatments with intravenous anti-viral medications, or patients requiring prolonged admissions to the hospital. The medicines can also have side effects like damage to the kidneys or reduction in the blood counts, so in this study we are trying to find an easier way to treat these infections.