|Classification and external resources|
A viral disease (or viral infection or infectious disease), occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.
Basic structural characteristics, such as genome type, virion shape and replication site, generally share the same features among virus species within the same family.
There are five 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.
There is one family of partly double-stranded DNA viruses: Hepadnaviridae. These viruses are enveloped.
There is one family of single-stranded DNA viruses that infect humans: Parvoviridae. These viruses are non-enveloped.
There are seven positive single-stranded RNA families: three non enveloped (Astroviridae, Caliciviridae and Picornaviridae) and four enveloped (Coronoviridae, Flaviviridae, Retroviridae and Togaviridae). All the non-enveloped families have icosahedral nucleocapsids.
There are six negative single-stranded RNA families: Arenaviridae, Bunyaviridae, Filoviridae, Orthomyxoviridae, Paramyxoviridae and Rhabdoviridae. All are enveloped with helical nucleocapsids.
There is one family with a double-stranded RNA genome: Reoviridae.
There is one additional virus (Hepatitis D virus) which has not yet been assigned to a family but is clearly distinct from the other families infecting humans.
There is one family and one genus of 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.
Useful rules of thumb
Among the human infecting families there are a number of rules that may assist physicians and medical microbiologists/virologists.
As a general rule, DNA viruses replicate within the 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.
Four families have segmented genomes: Bunyaviridae, Orthomyxoviridae, Arenaviridae and Reoviridae (acronym BOAR). All are RNA viruses.
Only one family of enveloped viruses causes gastroenteritis (Coronaviridae). All other viruses associated with gastroenteritis are non enveloped.
These are tables of the clinically most important viruses.
The clinical characteristics of viruses may differ substantially among species within the same family:
In 2010 it was reported that the presence of a tobamovirus (Pepper mild mottle virus) in the stool was associated with clinical disease and a specific immune response. If this association can be confirmed it is the first known case of disease caused by a virus previously considered pathogenic only to plants.
Diagnosis and treatment
Viral disease is usually detected by clinical presentation, for instance severe muscle and joint pains preceding fever, or skin rash and swollen lymph glands. Laboratory investigation is not directly effective in detecting viral infections, because they do not themselves increase the white blood cell count. Laboratory investigation may be useful in diagnosing associated bacterial infections, however. Viral infections are commonly of limited duration, so treatment usually consists in reducing the symptoms; antipyretic and analgesic drugs are commonly prescribed.
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