Polyomavirus is the sole genus of viruses within the family Polyomaviridae. Ludwik Gross discovered the first polyomavirus, Murine polyomavirus, in 1963. Many polyomaviruses infect birds and mammals and have been extensively studied as tumor viruses in humans and animals. Polyomaviruses are DNA-based, small, and icosahedral in shape. They can potentially cause tumors and often persist as latent infections in a host without causing disease.

There are three major clades or genetically-related groups: A SV40 clade, an avian clade, and the murine polyomavirus clade. There are six that have been found to infect humans. Most of these infections appear to cause little or no symptoms and commonly infect children and young adults. The viruses are persistent throughout life for most adults. SV40 replicates in the kidneys of monkeys without causing disease, but causes sarcomas in hamsters. It is under controversy whether it can cause disease in humans.

There is no widely accepted evidence that the virus is present in human cancer. It is also thought unlikely that SV40 is circulating widely among humans and that it is a major cause for human cancers.
Viral attachment, entry, and uncoating occur prior to genome replication. Viral protein 1 mediates the attachment of polyomaviruses to host cells. Anti-VP1 antibodies prevent the binding of polyomavirus to host cells. Diagnosis occurs after primary infection as it is either asymptomatic or sub-clinical. Antibody assays are often used to detect presence of antibodies against individual viruses. Three diagnostic tests are used: urine cytology, quantification of the viral load in both urine and blood, and a renal biopsy.

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