A virus can depend on another virus to reproduce
A virophage is a virus that multiplies only when a much larger virus is already infecting a cell. The larger virus may build a viral factory inside that cell: a concentrated workspace where it copies its genetic material and assembles new virus particles. The virophage uses parts of this setup to make copies of itself.
It does not burrow into a fully formed virus particle. Instead, both viruses operate inside the same infected cell, where the virophage can tap into the machinery the larger virus has established.
The smaller parasite can disrupt the larger one
Because the virophage draws on the giant virus’s replication system, its presence can interfere with the larger virus’s reproduction. Some of the factory’s resources and activity are diverted toward making virophages, leaving fewer opportunities for the giant virus to produce its own offspring. The virophage, meanwhile, benefits from a ready-made production site it could not create on its own.
This is why the relationship is sometimes described as a parasite nested within another parasite’s operation. The giant virus is exploiting a host cell; the virophage is exploiting the giant virus’s foothold in that cell. The result is a chain of dependence in which one virus’s attempt to multiply creates an opening for another.
