What term describes a virus that requires a co-infector for infection establishment?

A satellite virus is defined as a virus that is dependent on another virus for its replication and infection process. These satellite viruses cannot infect host cells on their own; they rely on a helper virus for key functions. The helper virus provides the necessary proteins and machinery for the satellite virus to replicate and form new virions. This unique dependency distinguishes satellite viruses from other types of viruses, which can usually infect host cells independently.

In contrast, a prophage is a form of a bacteriophage that has integrated its genetic material into the host bacterium's DNA, becoming a part of it, but it does not require another virus for its establishment. A viroid is a small infectious agent made of RNA that affects plants and does not require a helper virus for its propagation. A retrovirus is a type of virus that reverse transcribes its RNA genome into DNA; it does not inherently require another virus for infection establishment.

Hence, the term "satellite virus" appropriately describes the virus that necessitates a co-infector for establishing an infection.