Viruses in the family Retroviridae are found in a wide variety of vertebrate hosts. Enveloped virions are 80-100 nm in diameter with an inner core containing the viral genome and replicative enzymes. Core morphology is often characteristic for viruses within the same genus. Replication involves reverse transcription and integration into host cell DNA, resulting in a provirus. Integration into germline cells can result in a heritable provirus known as an endogenous retrovirus. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Retroviridae, which is available at ictv.global/report/retroviridae.

The Polyomaviridae is a family of small, non-enveloped viruses with circular dsDNA genomes of approximately 5 kbp. The family includes four genera whose members have restricted host range, infecting mammals and birds. Polyomavirus genomes have also been detected recently in fish. Merkel cell polyomavirus and raccoon polyomavirus are associated with cancer in their host; other members are human and veterinary pathogens. Clinical manifestations are obvious in immunocompromised patients but not in healthy individuals. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Polyomaviridae, which is available at www.ictv.global/report/polyomaviridae.

The Togaviridae is a family of small, enveloped viruses with single-stranded, positive-sense RNA genomes of 10-12 kb. Within the family, the genus Alphavirus includes a large number of diverse species, while the genus Rubivirus includes the single species Rubella virus. Most alphaviruses are mosquito-borne and are pathogenic in their vertebrate hosts. Many are important human and veterinary pathogens (e.g. chikungunya virus and eastern equine encephalitis virus). Rubella virus is transmitted by respiratory routes among humans. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Togaviridae, which is available at www.ictv.global/report/togaviridae.

The Papillomaviridae is a family of small, non-enveloped viruses with double-stranded DNA genomes of 5 748 to 8 607 bp. Their classification is based on pairwise nucleotide sequence identity across the L1 open reading frame. Members of the Papillomaviridae primarily infect mucosal and keratinised epithelia, and have been isolated from fish, reptiles, birds and mammals. Despite a long co-evolutionary history with their hosts, some papillomaviruses are pathogens of their natural host species. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Papillomaviridae, which is available at http://www.ictv.global/report/papillomaviridae.