More than a cause of a simple infection, viruses are often involved in the development of serious diseases. Such is the case with liver cancer, which often develops in an organ that has been weakened by hepatitis B or C virus. Researchers at Inserm, the Paris Public Hospitals (AP-HP), Paris Descartes University, Paris 13 University (USPC), and Paris Diderot University have just identified the role of a new virus, hitherto unsuspected, in the occurrence of a rare type of liver cancer.