Serology testing, including EBV seroprevalence, is always done in all participating sites during the pre-transplant evaluation (before conditioning chemotherapy) in both donors and recipients. Serology testing is the test that is used to confirm the presence of the virus in the blood. For grafts provided by external sites, donor blood samples are always available and will allow for serology testing at the laboratory center where the transplantation is performed. Serology testing will not be done on cord-blood donor as it is virtually always negative for EBV. Donors and recipients will be classified according to their pre-transplant serology status as: 1) having past infection (VCA and EBNA IgG titers > 10), 2) having recent infection or being immune suppressed (VCA IgG titers > 10 and EBNA IgG titers < 10), 3) having reactivated infection (VCA, EBNA and EA IgG titers > 10), or 4) being naïve (no serological sign of prior infection).