This sort of therapy has been used previously to treat Hodgkin or non-Hodgkin lymphomas that show proof of infection with Epstein-Barr virus (EBV), the virus that causes infectious mononucleosis ("mono" or the "kissing disease"). EBV is found in cancer cells of up to half of all patients with Hodgkin's and non-Hodgkin lymphoma. This suggests that it may play a role in causing lymphoma. The cancer cells infected by EBV are able to hide from the body's immune system and escape being killed. Investigators tested whether special white blood cells, called T cells, that were trained to kill EBV-infected cells could affect these tumors, and in many patients it was found that giving these trained T cells caused a complete or partial response.