Polyomaviridae is a family of viruses. Mammals and birds serve as natural hosts. There are currently 13 species in this family, contained within a single genus, Polyomavirus, whose type species is Simian virus 40. Not all members of the family infect humans, but among those that do, several are associated with human disease: JC virus with progressive multifocal leukoencephalopathy, BK virus with mild respiratory infection, 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).