The Epstein-Barr virus (EBV), which predominantly infects B-cells, has certain novel features, particularly its ability to remain latent for long periods and its capacity for transformation. Host immune response is complex, varying in intensity and quality. Disease expression results from this conflict between the host and the virus. Clinical manifestations are thus quite variable, ranging from asymptomatic infection, or various aspects of infectious mononucleosis, to malignant lymphoproliferation. This is a perfect illustration of the transition between benign and malignant reactive lymphoproliferation and of the organism's capacity to produce a more or less rapid and effective response to a potentially malignant disorder.