A viral disease (or viral infection) occurs when an organism's body is invaded by

pathogen In biology, a pathogen (, "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 Germ theory of d ...

ic viruses, and infectious virus particles (virions) attach to and enter susceptible cells. Examples include the

common cold The common cold, or the cold, is a virus, viral infectious disease of the upper respiratory tract that primarily affects the Respiratory epithelium, respiratory mucosa of the human nose, nose, throat, Paranasal sinuses, sinuses, and larynx. ...

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

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic. The symptoms of COVID‑19 can vary but often include fever ...

, 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 Adenoviruses (members of the family (biology), family ''Adenoviridae'') are medium-sized (90–100 nanometer, nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. ...

'', '' 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 Parvoviruses are a family of animal viruses that constitute the family ''Parvoviridae''. They have linear, single-stranded DNA (ssDNA) genomes that typically contain two genes encoding for a replication initiator protein, called NS1, and the pr ...

''. These viruses are non-enveloped. * Positive single-stranded

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

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 ''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 virus ...

''. 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 The cell nucleus (; : nuclei) is a membrane-bound organelle found in eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have #Anucleated_cells, ...

while RNA viruses replicate within the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

. 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

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s: ''Bunyavirus'', '' Flavivirus'', and '' Togavirus''. Some Reoviruses are transmitted from arthropod vectors. All are RNA viruses. * One family of enveloped viruses causes

(''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:

References

External links