The cause of NHL is unknown. Exposures to viruses and immunosuppression are thought to be related to NHL. Organ transplantation, a history of cancer treated with radiation, acquired immune deficiencies, and autoimmune disorders are considered risk factors. Some believe that both Hodgkin's disease and NHL result from an immune defect or from the activation of an oncogenic virus. One form of NHL, Burkitt's lymphoma, seems to be related to a herpes virus (Epstein-Barr virus). Exposure to nuclear explosions or reactor accidents, certain pesticides and herbicides, and chemicals (benzene, lead, paint thinner, and formaldehyde) may place patients at risk.