Viral infection is one of the major causes of morbidity and mortality in patients who receive bone marrow transplantation (BMT) from unrelated or mismatched donors. This increased risk of infection relates to a number of factors including the immunosuppressive regimens these patients receive, delayed immune recovery and the greater genetic disparity between donor and recipient that result in defective interactions between antigen presenting cells and immune system effector cells. In most cases viral infection post BMT results from reactivation of latent virus and CMV, EBV and adenoviruses (Ad) are the commonest viral pathogens causing disease after transplant.