A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.[1]
| Viral disease | |
|---|---|
| Other names | Viral infection |
 | |
| Transmission electron micrograph of Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virions | |
| Specialty | Infectious disease |
Examples are the common cold, gastroenteritis and pneumonia.[2]
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.[citation needed]
- 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.[citation needed]
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.[citation needed]
- 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.[3]
- 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:[citation needed]
- 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:
