virophage

thumb|upright=1.8| (A) When the host cell is only infected by a giant virus, the latter establishes a cytoplasmic virus factory to replicate and generates new virions, and the host cell is most likely [[lysed at the end of its replication cycle.(B) When the host cell is co-infected with a giant virus and its virophage, the latter parasitizes the giant virus's virus factory. The presence of virophages could seriously impact the infectivity of the giant virus by decreasing its replication efficiency and increasing the survival of the host cell.(C) When the giant virus genome is parasitized by a provirophage, the latter is expressed during the giant virus replication. The virophage is produced from the giant virus factory and inhibits the giant virus replication, thus increasing the host cell survival. ]] thumb|upright=1.8| (A) The replication of virophages is supposed to occur entirely in the virus factory of its giant virus host, depending on the giant virus expression/replication complex.(B) Satellite viruses initiate the expression and replication of their genomes in the nucleus using the host cell's machinery and then go to the cytoplasm. In the cytoplasm, the satellite virus hijacks the [[morphogenesis machinery of its helper virus to produce its progeny.]]

Virophages are small, double-stranded DNA viruses that require the co-infection of another virus. The co-infecting viruses are typically giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for their own replication. One of the characteristics of virophages is that they have a parasitic relationship with the co-infecting virus. Their dependence upon the giant virus for replication sometimes results in reduced virulence of the giant viruses. Many virophage genomes are integrated into the genomes of single-celled eukaryotes, and they reactivate upon infection of a giant virus, thereby providing a form of inducible antiviral defense. The virophage may improve the recovery and survival of the host organism. Most known virophages are classified within the class Virophaviricetes and are associated with giant viruses of the family Mimiviridae. Virophages are associated with a range of other host viruses, however, including insect poxviruses.