In order to demonstrate definitively that virus from transfused blood products can be linked to severe complications such as high and increasing viral load EBV infection/PTLD, we will genotype the EBV strain from these patients. Blood units administered to these patients will be traced back to the donors who in turn (with consent) will be serologically assessed for EBV, and all seropositive donors will have their EBV strain genotyped for comparison to the patient's strain. Many blood donors per case will have to be traced as recipients receive blood products from many blood donors (1 unit of RBC is provided by 1 donor, 1 unit of plasma is provided by 1 donor and 1 unit of platelet is provided by 4 donors). In average, we expected that about 25 donors will have to be traced per case (estimated with pilot data; Trottier et al, 2012). We are used in tracing blood donors of graft recipients as reported in Alfieri et al. (1996).