Like other polyomaviruses, SV40 is a DNA virus that is found to cause tumors in humans and animals, but most often persists as a latent infection.

[3] The hypothesis that SV40 might cause cancer in humans was a particularly controversial area of research, fuelled by the historical contamination of some batches of polio vaccine with SV40 in the 1950s and 1960s.

[12][13][14] SV40 consists of an unenveloped icosahedral virion with a closed circular double-stranded DNA genome[15] of 5.2 kb.

Resident ER proteins induce structural changes to initiate a disassembly process of the VP1 capsid shell.

SV40 then exploits components of Endoplasmic-reticulum-associated protein degradation machinery to penetrate the ER membrane and get extracted into the cytosol.

[20] Inside the cell nucleus, the cellular RNA polymerase II acts to promote early gene expression.

A putative late protein VP4 has been reported to act as a viroporin facilitiating release of viral particles and resulting in cytolysis;[21][22] however the presence and role of VP4 have been disputed.

Yamamato and Shimojo observed MR when SV40 virions were irradiated with UV light and allowed to undergo multiple infection of host cells.

In contrast, when multiple viral genomes infected a host cell, psoralen-induced DNA cross-links were repaired; that is, MR occurred.

[citation needed] SV40 was first identified by Ben Sweet and Maurice Hilleman in 1960 when they found that between 10 and 30% of polio vaccines in the US were contaminated with SV40.

[29] In 1962, Bernice Eddy described the SV40 oncogenic function inducing sarcoma and ependymomas in hamsters inoculated with monkeys cells infected with SV40.

[32] SV40 has become a totemic subject among anti-vaccination activists, where its presence in contaminated vaccine is accused of being a cause of a cancer "epidemic" and of being responsible for HIV/AIDS.