Viral disease

Examples include the common cold, gastroenteritis, COVID-19, the flu, and rabies.

Structural characteristics

Basic structural characteristics, such as genome type, virion shape and replication site, generally share the same features among virus species within the same family.

• Double-stranded DNA families: three are non-enveloped (Adenoviridae, Papillomaviridae and Polyomaviridae) and two are enveloped (Herpesviridae and Poxviridae). All of the non-enveloped families have icosahedral capsids.
• Partly double-stranded DNA viruses: Hepadnaviridae. These viruses are enveloped.
• One family of single-stranded DNA viruses infects humans: Parvoviridae. These viruses are non-enveloped.
• Positive single-stranded RNA families: three non-enveloped (Astroviridae, Caliciviridae and Picornaviridae) and four enveloped (Coronaviridae, Flaviviridae, Retroviridae and Togaviridae). All the non-enveloped families have icosahedral nucleocapsids.
• Negative single-stranded RNA families: Arenaviridae, Bunyaviridae, Filoviridae, Orthomyxoviridae, Paramyxoviridae and Rhabdoviridae. All are enveloped with helical nucleocapsids.
• Double-stranded RNA genome: Reoviridae.
• The Hepatitis D virus has not yet been assigned to a family, but is clearly distinct from the other families infecting humans.
• Viruses known to infect humans that have not been associated with disease: the family Anelloviridae and the genus Dependovirus. Both of these taxa are non-enveloped single-stranded DNA viruses.

Pragmatic rules

Human-infecting virus families offer rules that may assist physicians and medical microbiologists/virologists.

As a general rule, DNA viruses replicate within the cell nucleus while RNA viruses replicate within the cytoplasm. Exceptions are known to this rule: poxviruses replicate within the cytoplasm and orthomyxoviruses and hepatitis D virus (RNA viruses) replicate within the nucleus.

• Segmented genomes: Bunyaviridae, Orthomyxoviridae, Arenaviridae, and Reoviridae (acronym BOAR). All are RNA viruses.
• Viruses transmitted almost exclusively by arthropods: Bunyavirus, Flavivirus, and Togavirus. Some Reoviruses are transmitted from arthropod vectors. All are RNA viruses.
• One family of enveloped viruses causes gastroenteritis (Coronaviridae). All other viruses associated with gastroenteritis are non-enveloped.

Baltimore group

This group of analysts defined multiple categories of virus. Groups:

• I - dsDNA
• II - ssDNA
• III - dsRNA
• IV - positive-sense ssRNA
• V - negative-sense ssRNA
• VI - ssRNA-RT
• VII - dsDNA-RT

Clinical characteristics

The clinical characteristics of viruses may differ substantially among species within the same family: