Patients with chronic active Epstein-Barr virus (CAEBV) have (a) an illness that began as a primary infection with EBV or markedly elevated titers of antibodies to EBV, (b) histological evidence of organ disease, and (c) elevated EBV DNA levels in the blood or EBV DNA or proteins in affected tissues. This is primarily a disease of children and young adults. Many of these patients develop hypogammaglobulinemia or pancytopenia and death is frequently due to opportunistic infections or B or T cell lymphoproliferative disease. The primary goal of this study is to identify genetic mutations responsible for CAEBV. The secondary goal of this study is to learn more about the natural history of CAEBV. Blood samples or available tissues (e.g. previous biopsy or autopsy material) from patients with CAEBV and their relatives will be analyzed to identify disease genes that are mutated in this population. In addition, some of the patients will be invited to come for evaluation at the NIH Clinical Center. At that time medically indicated studies of disease progression will be performed. Serial virologic and immunologic studies will be performed. Eligible relatives of patients will be studied to compare the gene(s) associated with CAEBV in patients with the gene(s) in their relatives. Knowledge gained from this study has the potential of providing insights into the immunologic control of EBV infections. In addition, identification of the molecular mechanisms for CAEBV may provide insights leading to improved treatments for this disease.