Wikipedia
Polyomaviridae is a family of viruses whose natural hosts are primarily mammals and birds. As of the most recent (2015) taxonomy release by the International Committee on Taxonomy of Viruses, there were 73 recognized species in this family contained within four genera, as well as three species that could not be assigned to a genus. Of these, 13 species are known to infect humans. Most of these viruses, such as BK virus and JC virus, are very common and typically asymptomatic in most human populations studied. However, some polyomaviruses are associated with human disease, particularly in immunocompromised individuals; BK virus is associated with nephropathy in renal transplant and non-renal solid organ transplant patients, JC virus with progressive multifocal leukoencephalopathy, and Merkel cell virus with Merkel cell cancer.
Some members of the family are oncoviruses, meaning they can cause tumors; they often persist as latent infections in a host without causing disease, but may produce tumors in a host of a different species, or in individuals with ineffective immune systems. The family was first discovered due to its oncogenic properties; some members of the family, most prominently murine polyomavirus, have been extensively studied in the laboratory to understand the mechanism by which they induce carcinogenesis. The name polyoma refers to the viruses' ability to produce multiple (poly-) tumors (-oma).