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 ...

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

es, 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 Rabies is a viral disease that causes encephalitis in humans and other mammals. It was historically referred to as hydrophobia ("fear of water") because its victims panic when offered liquids to drink. Early symptoms can include fever and abn ...

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 Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

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 ''Papillomaviridae'' is a family of non- enveloped double-stranded DNA viruses whose members are known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all car ...

'' and ''

Polyomaviridae ''Polyomaviridae'' is a family of DNA viruses whose natural hosts are mammals and birds. As of 2024, there are eight recognized genera. Fourteen species are known to infect humans, while others, such as Simian Virus 40, have been identified i ...

'') and two are enveloped (''

Herpesviridae ''Orthoherpesviridae'', previously named and more widely known as ''Herpesviridae'', is a large family of DNA viruses that cause infections and certain diseases in animals, including humans. The members of this family are commonly known as herp ...

'' and ''

Poxviridae ''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 22 genera that are assigned to two subfamilies: ''Chordopoxvirinae'' and ''Entomopoxvirinae''. ''Entomopoxvirinae'' ...

''). 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 Protomer (structural biology), protomers. The observable 3-dimensional morpholog ...

. * Partly

double-stranded DNA viruses A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and t ...

: ''

Hepadnaviridae ''Hepadnaviridae'' is a family of viruses. Humans, apes, and birds serve as natural hosts. The family contains five genera. Its best-known member is hepatitis B virus. Diseases associated with this family include: liver infections, such as hep ...

''. These viruses are enveloped. * One family of

single-stranded DNA viruses A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and t ...

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 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 ...

'' 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 nm ...

'') and four enveloped (''

Coronaviridae ''Coronaviridae'' is a family (biology), family of Viral envelope, enveloped, positive-strand RNA viruses which infect fish, amphibians, birds, and mammals. The group includes the subfamilies ''Letovirinae,'' ''Orthocoronavirinae'', and ''Pitovir ...

'', ''

Flaviviridae ''Flaviviridae'' is a family of Viral envelope, enveloped positive-strand RNA viruses which mainly infect mammals and birds. They are primarily spread through arthropod vector (epidemiology), vectors (mainly ticks and mosquitoes). The family gets ...

'', ''

Retroviridae 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. After invading a host cell's cytoplasm, the virus uses its own reverse transcriptase ...

'' and ''

Togaviridae {{Commonscat, Viruses by family This category is for articles about virus families (or redirects to such articles). There should be no subcategories. families Families (biology) ...

''). All the non-enveloped families have icosahedral nucleocapsids. * Negative single-stranded RNA families: '' Arenaviridae'', ''

Bunyaviridae ''Bunyaviricetes'' is a class of segmented negative-strand RNA viruses with mainly tripartite genomes. Member viruses infect arthropods, plants, protozoans, and vertebrates. The name ''Bunyaviricetes'' derives from Bunyamwera, where the virus B ...

'', ''

Filoviridae ''Filoviridae'' () is a International Committee on Taxonomy of Viruses, family of RNA virus#Group_V_–_negative-sense_ssRNA_viruses, single-stranded negative-sense RNA viruses in the order (biology), order ''Mononegavirales''. Two members of the ...

'', ''

Orthomyxoviridae ''Orthomyxoviridae'' () is a family of negative-sense single-stranded RNA virus, negative-sense RNA viruses. It includes nine genus, genera: ''Influenza A virus, Alphainfluenzavirus'', ''Influenza B virus, Betainfluenzavirus'', ''Influenza C vir ...

'', ''

Paramyxoviridae ''Paramyxoviridae'' (from Ancient Greek, Greek ''para-'' “by the side of” and ''myxa'' “mucus”) is a family of negative-strand RNA viruses in the order ''Mononegavirales''. Vertebrates serve as natural hosts. Diseases associated with this ...

'' 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 ''Reovirales'' is an order of double-stranded RNA viruses. Member viruses, called reoviruses, have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented geno ...

''. * The

Hepatitis D virus Hepatitis D is a type of viral hepatitis caused by the hepatitis delta virus (HDV). HDV is one of five known hepatitis viruses: A, B, C, D, and E. HDV is considered to be a satellite (a type of subviral agent) because it can propagate only in ...

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 (biology), family ''Parvoviridae''; they are Group II viruses according to the Baltimore classific ...

''. 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: ''

'', ''

'', '' Arenaviridae'', and ''

'' (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 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 deoxy ...

* 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. After invading a 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

{{DEFAULTSORT:Virus Disease Pathogenic microbes

Structural characteristics

Pragmatic rules

Baltimore group

Clinical characteristics

See also

References

External links