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A viral disease (or viral infection) occurs when an organism's body is invaded by
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
ic
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
es, 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 Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from thei ...
'', '' Papillomaviridae'' and ''
Polyomaviridae ''Polyomaviridae'' is a family of viruses whose natural hosts are primarily mammals and birds. As of 2020, there are six recognized genera and 117 species, five of which are unassigned to a genus. 14 species are known to infect humans, while oth ...
'') and two are enveloped (''
Herpesviridae ''Herpesviridae'' is a large family of DNA viruses that cause infections and certain diseases in animals, including humans. The members of this family are also known as herpesviruses. The family name is derived from the Greek word ''ἕρπει� ...
'' and ''
Poxviridae ''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. There are currently 83 species in this family, divided among 22 genera, which are divided into two subfamilies. Diseases associated wit ...
''). 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 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 Picornaviruses are a group of related nonenveloped RNA viruses which infect vertebrates including fish, mammals, and birds. They are viruses that represent a large family of small, positive-sense, single-stranded RNA viruses with a 30  ...
'') 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 ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ...
'', Paramyxoviridae'' Paramyxoviridae'' and ''
Rhabdoviridae ''Rhabdoviridae'' is a family of negative-strand RNA viruses in the order '' Mononegavirales''. Vertebrates (including mammals and humans), invertebrates, plants, fungi and protozoans serve as natural hosts. Diseases associated with member ...
''. All are enveloped with helical nucleocapsids. * Double-stranded RNA genome: ''
Reoviridae ''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-layere ...
''. * 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 A microbiologist (from Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of microscopic organisms such as bacteria, algae, fungi, and some types of para ...
/
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, their ...
. 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 In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
. 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 ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ...
'', '' Arenaviridae'', and ''
Reoviridae ''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-layere ...
'' (acronym BOAR). All are RNA viruses. * Viruses transmitted almost exclusively by
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s: ''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 dehydr ...
(''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 ...
* VII - dsDNA-RT


Clinical characteristics

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


See also

* List of latent human viral infections *
Pathogenic bacteria Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and are often beneficial but others can cause infectious diseases. The number of t ...


References


External links

{{DEFAULTSORT:Virus Disease Pathogenic microbes