A common misconception is that the word "Mutagenesis" derives from "Mutation" and "Gene". In fact, mutagenesis refers to the Generation of Mutations.
Virus insertional mutagenesis is only possible with a replication competent virus. The virus inserts a gene (known as a viral onocogene) normally near the cellular myc (c-myc)gene. The c-myc gene is normally turned off in the cell, however when it is turned on it is able to push the cell into the G1 phase of the cell cycle and cause the cell to begin replication which allows the viral gene to be replicated. After many replications where the viral gene stays latent tumours begin to grow. These tumours are normally derived from one mutated/ transformed cell (clonal in origin). Avian leukosis virus is an example of a disease caused by insertional mutagenesis, where newly hatched chicks get infected with the virus and after a month a transformation event occurs and tumours begin to appear in their bursa of fabricus (like the human thymus). This viral gene insertion is also known as a promoter insertion as it drives the expression of the c-myc gene. There is an example of an insertional mutagenesis event caused by a retrotransposon in the human genome where it causes Fukuyama-type muscular dystrophy