Virophages are double-stranded DNA viruses with 20-30 kilobase pair (kbp) long genomes that replicate inside single-celled eukaryotes such as amoebae and flagellates. Their propagation depends on a co-infecting giant DNA virus of the family Mimiviridae, because virophages replicate within the cytoplasmic virion factory of the giant DNA virus. Virophages act as parasites of giant viruses and increase the survival rates of their host cells in the presence of giant viruses. In this talk, I will show that the mavirus virophage is able to integrate its genome into the nuclear genome of its host, the marine flagellate Cafeteria roenbergensis. Integrated provirophages are then reactivated and produce infectious virions in response to infection by the giant virus CroV. Virophages thus protect their hosts from lytic giant viruses, while integration into the host genome enables the virophage to persist for prolonged periods of time.