Cells can be made cancerous by viruses, even those that are considered harmless.
A virus can cause massive chromosomal instability by fusing cells, increasing the rate of chromosome mutations. Fusion abruptly unites two or three cells under the same membrane
At least 18 of 29 virus families that infect human cells have species that fuse cells.
Chromosomal instability is characteristic of most human cancers. All malignant tumor types have been shown to contain chromosomal aberrations. Inactivating these viruses would help to decrease the incidence of cancer. [story]