It is not known what causes some normal blood cells to become malignant and cause systemic ALCL. There have been suggestions that an infection with a very common virus (Epstein-Barr virus) years previously may contribute to some cases of ALCL. This is not yet proven. Since the virus is very common and most people in the USA and Europe have been exposed to the virus, it is not known whether and why it may lead to ALCL in just a few patients. A genetic mutation in the tumor cells also seems to contribute to the origin of some cases. However this mutation is only present in half of all cases.