Foundational Virology

Every virus, from a common-cold rhinovirus to a haemorrhagic-fever agent, is built and behaves according to the same small set of principles. With only a handful of genes it must specify a particle that protects its genome, deliver that genome into a living cell, present it to the host ribosome as messenger RNA, copy it, and escape to infect again. Foundational Virology sets out that shared machinery: how viruses are classified and named, how the virion is assembled, how the genome is replicated, and how viral populations evolve. The particulars move as sequencing redraws relationships and taxonomy is revised, but the underlying logic is what makes the rest of the field intelligible.

Two ideas recur throughout. The first is genetic economy: a virus carries only information, building its coat from a few proteins copied many times and borrowing the host’s ribosomes and energy for everything else. The second is the constraint that every genome, whatever its form, must be converted into positive-sense messenger RNA before it can be read; the Baltimore classification is the map of the seven routes different genomes take to get there.

The topic teaches the systems of virology rather than the agents. The identity of any individual virus, its current taxonomic name, family, genome and Baltimore class, lives in that virus’s own profile in the A–Z index; the systems here explain the machinery those profiles sit within. It is built from four articles.

→ See Classification and Nomenclature of Viruses for the taxonomic hierarchy and the International Committee on Taxonomy of Viruses (ICTV), the criteria that place a virus, the definition of a species, the rules of naming, the informal groupings clinicians use, and how a virus is shown to cause disease.

→ See Virion Structure and Composition for capsid symmetry, the envelope, genome architecture, and the stability rules that separate enveloped from naked viruses.

→ See Virus Replication for the Baltimore classification, the generic replication cycle and where antivirals act, and how viruses are grown and counted in the laboratory.

→ See Viral Evolution for mutation, recombination and reassortment, the quasispecies, antigenic variation, and the molecular clock that lets outbreaks be dated and traced.

The Baltimore classification

The organising scheme for the whole topic groups every virus by the route its genome takes to messenger RNA. The negative-sense and double-stranded RNA viruses cannot have their genome read on entry and so must carry their own polymerase inside the virion; a positive-sense RNA genome is itself messenger RNA and is translated the moment it arrives.

Class	Genome	Route to messenger RNA	Examples
I	Double-stranded DNA	Transcribed by host polymerase (poxviruses carry their own)	Herpesviruses, adenoviruses, poxviruses
II	Single-stranded DNA	Made double-stranded, then transcribed	Parvoviruses
III	Double-stranded RNA	Own polymerase transcribes from within the core	Reoviruses (rotavirus)
IV	Positive-sense RNA	Genome is messenger RNA, translated on entry	Picornaviruses, flaviviruses, coronaviruses
V	Negative-sense RNA	Own polymerase transcribes messenger RNA first	Influenza, paramyxoviruses, rhabdoviruses
VI	Positive-sense RNA, reverse-transcribing	Reverse-transcribed to DNA, integrated, then transcribed	Retroviruses (HIV)
VII	Double-stranded DNA, reverse-transcribing	Transcribed to an RNA intermediate, then reverse-transcribed to DNA	Hepadnaviruses (hepatitis B)

Key terms

The vocabulary that recurs across the topic, grouped by theme.

Classification and nomenclature:

Term	Definition
International Committee on Taxonomy of Viruses (ICTV)	The single body that sets viral taxonomy and the naming of taxa.
Taxonomic hierarchy	The 15-rank scheme from realm down to species; only genus and species assignment is obligatory.
Polyphyletic	Having several independent evolutionary origins, so viruses form no single tree of descent.
Polythetic	Defined by a set of shared properties, no single one of which is essential for membership.
Binomial species name	The current species format: a genus name plus a one-word epithet, such as Morbillivirus hominis for measles.
Vernacular name	The everyday working name of a virus (measles virus), lowercase and outside ICTV control.
Arbovirus	A virus that replicates within a blood-feeding arthropod before transmission to a vertebrate.
Henle–Koch postulates	The classical criteria for proving an agent causes a disease, later adapted for viruses and for the sequencing era.

Virion structure:

Term	Definition
Capsid	The protein coat enclosing the genome.
Nucleocapsid	The capsid together with the nucleic acid it encloses.
Envelope	A host-derived lipid bilayer, studded with virus-coded glycoprotein spikes, wrapping some virions.
Peplomer (spike)	A surface glycoprotein that binds the receptor and drives membrane fusion.
Matrix protein	A non-glycosylated layer lining the inner face of the envelope, giving the particle rigidity.
Icosahedral and helical	The only two true capsid symmetries; anything else is termed complex.
Triangulation number (T)	The multiple of 60 that sets how many subunits build an icosahedral capsid.
Capsomere (hexon, penton)	A surface unit of an icosahedral capsid, surrounded by six neighbours (hexon) or, at the vertices, five (penton).
Pleomorphic	Varying in size and shape from one particle to the next, as many enveloped viruses are.
Ligand and receptor	The binding molecule on the virus (ligand) and the molecule on the host cell it engages (receptor).
Ether sensitivity	Loss of infectivity on exposure to lipid solvents, the classic bench test for an envelope.

Genome and Baltimore class:

Term	Definition
Baltimore classification	The scheme grouping viruses into seven classes by the route from genome to messenger RNA.
Monopartite and multipartite genome	A genome carried as a single nucleic-acid molecule (monopartite) or divided across several separate segments (multipartite, or segmented), as in influenza and the reoviruses.
Positive- and negative-sense	Whether a single-stranded RNA genome reads directly as messenger RNA (positive) or must be transcribed first (negative).
Haploid and diploid	Viral genomes carry one copy of each gene, except the diploid retroviruses, which carry two.
RNA-dependent RNA polymerase (RdRp)	The enzyme that copies RNA from an RNA template, which host cells lack.
Reverse transcriptase	The enzyme that copies RNA into DNA, used by retroviruses and hepadnaviruses.

Replication cycle:

Term	Definition
Tropism	The set of tissues a virus infects, set largely by where its receptor is displayed.
Eclipse period	The interval after uncoating during which no infectious particle can be recovered.
One-step growth curve	The population time-course of a single synchronised round of infection.
Polyprotein	A single translated product cleaved by a viral protease into separate functional proteins.
Internal ribosome entry site (IRES)	An RNA element that lets translation begin within a message rather than only at its start.
Replication organelle	A membranous factory that concentrates the replication machinery and hides viral RNA from innate sensors.
Cytopathic effect	The visible damage (rounding, fusion or death) a virus causes in cultured cells.

Cultivation, counting and diversity:

Term	Definition
Plaque assay (plaque-forming unit)	Counting infectious virus as foci of dead cells in a monolayer, read in plaque-forming units.
Particle-to-infectivity ratio	The excess of physical particles over infectious ones, often greater than a thousand to one.
Quasispecies	The swarm of closely related genomes clustered around a consensus sequence.
Recombination and reassortment	The joining of sequences from different genomes; reassortment swaps whole segments and drives the antigenic shift of influenza.
Antigenic drift and shift	Gradual antigenic change by point mutation (drift) versus abrupt change by reassortment (shift).
Molecular clock	The roughly constant rate of sequence change that lets divergence times and outbreaks be dated.
Error catastrophe	Population collapse when the mutation rate is pushed past the error threshold.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Classification of Viruses; Virion Structure and Composition; Virus Replication. In: Fenner and White’s Medical Virology, 5th edition. Academic Press / Elsevier; 2017. Chapters 2 to 4. The readable backbone for the topic’s classification, structure and replication material.
• Pfeiffer JK, Condit RC, Schoggins JW; Harrison SC; Mercer J, Whelan SPJ, et al. Principles of Virology; Principles of Virus Structure; Virus Entry and Uncoating and Viral Replication Strategies. In: Fields Virology, 7th edition, Volume 4: Fundamentals. Wolters Kluwer; 2022 to 2023. The current, deeper treatment of the same material.
• International Committee on Taxonomy of Viruses. Virus Taxonomy: the Master Species List and online Taxonomy Browser (MSL41, ratified February 2026). ictv.global/taxonomy. The live, authoritative source for current viral taxonomy.