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.

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 A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may ...

. * 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 Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

families: three non-enveloped (''

Astroviridae ''Astroviridae'' is a family of non-enveloped ssRNA viruses that cause infections in different animals. The family name is derived from the Greek word ''astron'' ("star") referring to the star-like appearance of spikes projecting from the surfac ...

'', ''

Caliciviridae The ''Caliciviridae'' are a family of "small round structured" viruses, members of Class IV of the Baltimore scheme. Caliciviridae bear resemblance to enlarged picornavirus and was formerly a separate genus within the picornaviridae. They are p ...

'' and '' Picornaviridae'') and four enveloped ('' Coronaviridae'', '' Flaviviridae'', '' Retroviridae'' and '' Togaviridae''). All the non-enveloped families have icosahedral

nucleocapsids A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may ...

. * Negative single-stranded RNA families: '' Arenaviridae'', '' Bunyaviridae'', '' Filoviridae'', ''

Orthomyxoviridae ''Orthomyxoviridae'' (from Greek language, Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense single-stranded RNA virus, negative-sense RNA viruses. It includes seven genus, genera: ''Influenza ...

'', Paramyxoviridae'' 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 ''Dependoparvovirus'' (formerly ''Dependovirus'' or Adeno-associated virus group) is a genus in the subfamily ''Parvovirinae'' of the virus family ''Parvoviridae''; they are Group II viruses according to the Baltimore classification. Some depend ...

''. 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 Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, the ...

. As a general rule, DNA viruses replicate within the

cell nucleus The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...

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'', ''

'', '' Arenaviridae'', and '' Reoviridae'' (acronym BOAR). All are RNA viruses. * Viruses transmitted almost exclusively by arthropods: ''Bunyavirus'', '' Flavivirus'', and '' Togavirus''. Some

Reoviruses ''Reoviridae'' is a family of double-stranded RNA viruses. Member viruses have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented genome within multi-layered ...

are transmitted from arthropod vectors. All are RNA viruses. * One family of enveloped viruses causes

gastroenteritis Gastroenteritis, also known as infectious diarrhea and gastro, is an inflammation of the gastrointestinal tract including the stomach and intestine. Symptoms may include diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydra ...

(''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 A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase e ...

* VII - dsDNA-RT

Clinical characteristics

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

References

External links