General virology

Last updated: May 31, 2022

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A virus is an obligate intracellular parasite, meaning that it can only survive within a host cell and depends on it for replication and metabolic processes, e.g., protein synthesis. Viruses can be classified based on their genome (DNA or RNA) or other structural components, such as the capsid, the envelope, and the viral receptor proteins (spikes). The viral replication cycle occurs within the host cell and involves attachment to and penetration of the host cell, uncoating of the nucleic acid, replication of the nucleic acid, synthesis of virus proteins, assembly of the components, and release of new viruses via budding or cell lysis. The process of nucleic acid replication differs between DNA and RNA viruses. The host body has various physical and immunological defense mechanisms to inactivate and eliminate viruses. However, some viruses have the ability to persist in a dormant state within the host's body (e.g., Herpesviridae) after an active infection has resolved. The most important diagnostic tools in virology are serological testing and nucleic acid detection. This article provides an overview of the most common enveloped and nonenveloped RNA viruses and DNA viruses. For more details regarding the individual viruses, please see the corresponding articles.


  • A virus is an obligate intracellular parasite. Accordingly, it can only survive within a host cell and depends on it for replication and metabolic processes.
  • Virion: The infective form of a virus when present outside of cells, which consists of DNA or RNA, a protein capsid, and sometimes an envelope.

Viral structure

The most important viral components include:

Viral genome

To remember that REOvirus has a double-stranded RNA genome, think: “Double Rhymes are REOccuring all the time”

“A vegan INFLUENCER ate (8) 3 BUNnies and upset 10–12 REaders in 2 AREas: ”INFLUENZA virus has 8 segments, BUNyavirus has 3, REoviruses has 10–12, and AREnaviruses has 2.


Protein coat composed of capsomeres (aggregations of protomer subunits) that encloses the genome



To remember the +ssRNA viruses (TOGavirus, RETRovirus, HEPevirus, PICornavirus, CAlicivirus, FLAVivirus, and CORonavirus), think “2 Golden Retrievers are Heppily Picturing Cauliflower-Flavoured Corn dogs.”

To remember the RNA viruses that are naked (Picornaviridae, Reoviridae, Caliciviridae, and Hepeviridae), think: ”Don't Run around naked, put on some PRetty ClotHes.”

To remember that ADEnovirus, PAPillomavirus, POLyomavirus, and PARvovirus are DNA viruses without an envelope, think of “Without ADEquate PAPers, the POLice will Deny PARole.”

To remember that pOXvirus, hepatitis B virus and HERpesvirus are enveloped DNA viruses, think of “A bOXer Dog will never B a HERbivore.”

Viral life cycle

Viruses replicate by synthesizing and assembling their individual components within the host cell.

  1. Attachment to the host cell: viruses use host cell surface proteins and receptors for entry (see receptors used by viruses below)
  2. Penetration into the host cell
  3. Uncoating of the nucleic acid
  4. Replication of the nucleic acid and formation of virus proteins by transcription and translation (in retroviruses, RNA is initially transcribed into DNA)
  5. Assembly of virus components
  6. Viral release

The period between uncoating in the host cell and production of recognizable virus particles is known as the eclipse period.

Receptors used by viruses

Viruses use host cell surface proteins and receptors to attach and penetrate the cells.

“The rhino knocked over mI CAMera”: rhinovirus enters cells via ICAM-1.


Mechanisms by which viruses cause infection in the host:

Course of viral infection

  • Abortive (no viral replication or cell damage)
  • Acute
  • Chronic
  • Persistent
    • Latent viral infection (virus is inactive; no replication): virus remains dormant in infected cells
    • Productive viral infection (viral replication occurs, dormant infection with few or no signs of infection)
    • Transforming viral infection (virus may or may not replicate): triggers malignant transformation (e.g., EBV, HPV)

Host defense mechanisms

The body has multiple defense mechanisms to inactivate and eliminate viruses. See “Innate immune system” and “Adaptive immune system.”

Interferon can be used to treat active hepatitis B and hepatitis C.

Viral genome replication

Viral genomic replication depends on the viral genome of the progenitor virus.

Viruses with DNA genomes (DNA viruses)

DNA viruses replicate in the nucleus of host cells (except Poxviridae, which carry their own DNA-dependent RNA polymerase).

To remember that HERpes, ADeno, POLyoma, POx, PARvo, HEPadna, and PAPilloma are DNA viruses, think: “I saw HER AD about a POLtergeist and POkemon PARty for HEPcats in the DAily PAPer.

Viruses with RNA genomes (RNA viruses)

RNA viruses replicate in cytoplasm of host cells (except Retroviridae and influenza viruses).

To remember that poxvirus is the only DNA virus that replicates outside the nucleus, think: “POX, Progeny is Outside the boX.”

To remember that every RNA virus replicates in the cytoplasm (except influenza and retrovirus), think: “aRe NA-VI RUStics CYTy slickers? Yes, except if INFLUENced by RETRO fashion!”

To remember that orthomyxovirus, paramyxovirus, arenavirus, filovirus, bunyavirus, and rhabdovirus are -ssRNA viruses, think: “ORTHOpedics PARAglide into an ARENA FILled with BUNnies that have RABies.”

Viral infectivity of nonenveloped viruses

The infectivity of naked viruses is determined by the genome.

Genetic diversification

Viral genetic exchange mechanisms
Process Mechanism
Recombination (viral)
  • Gene exchange between two chromosomes
    • Crossover between two regions of homologous base sequences
    • Results in progeny with genetic material from two parental viral strains
Reassortment (viral)
Complementation (viral)
  • Occurs in two different scenarios
    • Scenario 1: two mutated viruses from same/different family infect the same cell
    • Scenario 2
      • Mutated viral genome codes for a nonfunctional protein, a nonmutated viral genome codes for a functional protein.
      • The functional protein can be used by both mutated and nonmutated virus.
      • E.g., HBV codes for HBsAg, which is used by hepatitis D virus (HDV) as an envelope protein → HDV infection (without HBsAg, HDV cannot cause infection)
Phenotypic mixing (viral)
Phenotypic masking (transcapsidation)
Point mutations

Some viruses are DESParate: antigenic Drift → Epidemics, antigenic Shift → Pandemic.

The most important diagnostic tools in virology are serological testing and nucleic acid detection. To identify specific, localized increase in viral production, different biological materials should be analyzed and compared.

Viral family Capsid Genetic structure Important examples Diseases
  • Icosahedral
  • dsDNA
  • Linear
  • Icosahedral
  • Partially dsDNA
  • Circular
  • Hepatitis B (acute or chronic)
  • Complex
  • Mullouscipoxvirus genus
  • Orthopoxvirus genus
  • Variola virus
  • Smallpox: declared eradicated in 1980. The virus was eradicated by 1980 through widespread immunization with a live-attenuated vaccine and is no longer found outside of laboratories.
  • Cowpox

Viral family Capsid Genetic structure Important examples Diseases
  • Icosahedral
  • dsDNA
  • Linear
  • Adenovirus
    • More than 50 serotypes
    • Transmission via contaminated water or fecal-oral route
    • Different serotypes infect various cells
    • May persist after primary infection
  • dsDNA
  • Circular
  • Human papillomavirus (HPV)
    • Comprised of ∼ 100 genotypes
      • Low-risk subtypes: include HPV 1, 2, 6 and 11
      • High-risk subtypes: include HPV 16, 18, 31, and 33
    • Transmission mainly via sexual intercourse
    • Persistent after primary infection
    • Active HPV vaccination recommended for individuals 9–45 years of age
  • dsDNA
  • Circular
  • JC virus
    • Transmission usually occurs during childhood
    • Persistent after primary infection

To remember the diseases associated with polyomaviruses, think: “JC virus leads to a Junky Cerebrum (PML) and BK virus leads to Bad Kidneys (nephropathy in immunocompromised patients).”


Viral family Capsid Genetic structure Important examples Diseases
  • Helical
  • Linear
  • -ssRNA
  • Nonsegmented
  • Fusion protein (F protein) on surface: special virulence factor that causes fusion of respiratory epithelial cells → formation of multinucleated cells
  • Respiratory syncytial virus (RSV)
    • Transmission via airborne droplets
    • Highly contagious
    • Infects ciliated epithelial cells of the respiratory tract
    • Humans are sole host
    • Most commonly infects infants and children
  • Human metapneumovirus
    • Transmission via airborne droplets
    • Most commonly infects infants
  • Parainfluenza virus
    • Transmission via airborne droplets
    • Infects upper respiratory tract
    • Young children and infants are commonly affected
    • Relevant in both human and veterinary medicine
  • Hendra virus
    • Natural reservoir: flying fox
    • Transmission: via direct contact with bodily fluids of infected domestic animals (e.g., horses)

PaRaMyxovirus includes Parainfluenza, RSV, and Measles/Mumps/(human) Metapneumovirus.


Virus genus Capsid Genetic structure Important examples Diseases
  • Icosahedral
Flavivirus (Belong to the arboviruses)
  • Icosahedral
  • Tick-borne encephalitis virus
  • Dengue virus
    • Reservoir: humans
    • Vector: mosquitoes
    • Predominant occurrence in the Caribbean, South America, Southeast Asia, and Oceania
  • West Nile virus
  • West Nile encephalitis
  • St. Louis encephalitis virus
  • St. Louis encephalitis
  • Zika virus
    • Reservoir: primarily African monkeys and other primates
    • During outbreaks, human-to-vector-to-human transmission occurs
    • Vectors: mosquitoes (Aedes aegypti and Aedes albopictus)
    • Predominant occurrence: South America, Africa, and Southeast Asia

ARBOvirus is an acronym for ARthropod BOrne virus.


Viral genus Capsid Genetic structure Important examples Diseases
Influenza viruses
  • Helical

Other enveloped RNA viruses

Virus family Capsid Genetic structure Important examples Diseases
  • Helical
  • Lyssavirus
  • Australian bat lyssavirus: closely related to the rabies virus
  • Helical
  • Mild upper respiratory and gastrointestinal tract infection
  • Severe Acute Respiratory Syndrome (SARS; outbreak 2002)
  • Middle East Respiratory syndrome (MERS; outbreak 2012)
  • Coronavirus Disease 2019 (COVID-19; outbreak 2019/20)
  • Complex and conical (HIV)
  • Icosahedral (HTLV)
Bunyaviridae: recently reclassified as the order Bunyavirales
  • Helical
  • -ssRNA (3 segments)
  • Pseudocircular
  • Hantavirus
    • Different subtypes depending on geographical region
    • Reservoir: rodents
    • Routes of transmission
      • Aerogens by contaminated dust
      • Bite from an infected animal
  • Crimean-Congo hemorrhagic fever virus
    • Reservoir: animals, e.g., birds
    • Vector: ticks
  • La Crosse virus
  • La Crosse encephalitis
  • California encephalitis virus
  • California encephalitis
  • Rift Valley fever virus
  • Rift Valley fever
  • Helical
  • +ssRNA and -ssRNA (2 segments)
  • Circular
  • Lassa virus
    • Occurs mainly in West Africa
    • Reservoir: rats
  • Lassa fever
  • Lymphocytic choriomeningitis virus
    • Occurs mainly in Europe, Australia, Japan, North America, and South America
    • Reservoir: rats
  • Icosahedral
  • Western Equine Encephalitis virus
  • Western equine encephalitis
  • Eastern Equine Encephalitis virus
  • Eastern equine encephalitis [3]
  • Chikungunya virus
    • Mainly occurs in tropical and subtropical regions
    • Reservoir: monkeys
    • Vector: mosquito (Aedes aegypti, Aedes albopuctis)
  • Helical
  • Ebolavirus/Marburgvirus
  • Unknown
  • Icosahedral

To remember that HDV can only replicate in the presence of HBV, think: “HDV is Deficient without a Buddy (HBV).”


Viral family Capsid Genetic structure Important examples Diseases
  • Icosahedral
  • Echovirus
  • Hepatitis A virus (HAV)
    • High infection rate from infected water and food, particularly in subtropical and tropical regions
    • Transmission: fecal-oral
    • No chronicity
    • Prevention: inactivated vaccine for persons at risk and travelers to endemic regions.

To remember that COxsackie virus, HAV, POLiovirus, RHInovirus, and ECHOvirus are PICornaviridae, think: “The COps HAVe a POLice RHINO that ECHOS any song they PICk.”

Other nonenveloped RNA viruses

Viral genus Capsid Genetic structure Important examples Diseases
  • Icosahedral
  • Astrovirus
    • Infectious one day before and one day after clinical manifestation of disease
    • Transmission: fecal-oral
  • Icosahedral (double or triple layer)
  • dsRNA
  • Mulitsegmented (10–12 segments)
  • Linear
  • Rotavirus
  • Coltivirus
  • Icosahedral
  • Hepatitis E virus (HEV)
    • Reservoir: wild boars, domestic pigs
    • Transmission: fecal-oral
  • Icosahedral
  • Norovirus
    • Only occurs in humans
    • Transmission: fecal-oral and aerosols (e.g., during vomiting)
    • Highly contagious

COLTIvirus causes COLorado TIck fever.

  1. H H Hirsch, P Randhawa. BK Virus in Solid Organ Transplant Recipients. American Journal of Transplantation. 2001 .
  2. Gamblin SJ, Skehel JJ. Influenza hemagglutinin and neuraminidase membrane glycoproteins. J Biol Chem. 2010; 285 (37): p.28403-9. doi: 10.1074/jbc.R110.129809 . | Open in Read by QxMD
  3. Eastern Equine Encephalitis (EEE). Updated: October 7, 2020. Accessed: October 8, 2020.
  4. Chikungunya Virus. Updated: September 19, 2019. Accessed: October 8, 2020.
  5. Longmore M, Wilkinson IB, Davidson EH, Foulkes A, Mafi AR. Oxford Handbook of Clinical Medicine (2010). OUP Oxford ; 2010

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