Circoviruses infect swine, canine, mustelid, and avian species. They are part of the Circoviridae family and Circovirus genus. They are non-enveloped, circular, single-stranded DNA viruses. Most infections in both wild and domestic species are subclinical. However, circovirus infections are responsible for severe pathology in commercial swine operations. These viruses cause a range of gastrointestinal, neurological, dermatological, and respiratory signs in their hosts, and are most apparent in young animals. They are often found in conjunction with other pathogens in many species.