Polyomaviridae is a family of viruses. Mammals and birds serve as natural hosts. As of the most recent (2014) taxonomy release by the International Committee on Taxonomy of Viruses, there were 13 recognized species in this family, contained within a single genus, Polyomavirus, whose type species is Simian virus 40. There are 13 known polyomaviruses that 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 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. The name polyoma refers to the viruses' ability to produce multiple (poly-) tumors (-oma).