A family of viruses. Infection with a polyomavirus can sometimes lead to cancer, for example in patients with a weakened immune system.

Polyomavirus is the sole genus of viruses within the family Polyomaviridae. Polyomaviruses are DNA-based (double-stranded DNA,~5000 base pairs,circular genome), small (40-50 nanometers in diameter), and icosahedral in shape, and do not have a lipoprotein envelope. They are potentially oncogenic (tumor-causing); they often persist as latent infections in a host without causing disease, but may produce tumors in a host of a different species, or a host with an ineffective immune system.