Viral Pathogenesis

Viral pathogenesis is the study of how a virus infection becomes disease: the biological and molecular mechanisms, and the sequence in which they unfold, that carry a virus from the first exposure to the clinical illness it produces. The sequence is remarkably consistent and specific for each virus, which is what makes pathogenesis a subject that can be reasoned about rather than merely catalogued.

The topic turns on two ideas. The first is that an infection proceeds as an ordered journey through the host: a virus crosses a body surface, replicates, spreads through lymph, blood, or nerves to a target organ, injures it, and is shed to reach the next host. The second is that the outcome of that journey, anywhere from silent infection to death, is the balance between how virulent the virus is and how resistant the host is, struck afresh in every individual.

Disease itself arises in two principal ways, with a third that acts over a longer term. The virus may injure the cells it infects directly, or, for a virus that does not itself kill the cell, the host’s own immune response may inflict the damage, the route known as immunopathology. Over years, some infections instead drive a cell toward cancer, the work of the oncogenic viruses. The countermeasures a virus uses to survive the immune response, and so to persist, are the subject of viral immune evasion.

These articles work through the topic in turn.

→ See Viral Pathogenesis: an Overview for the journey itself: how a virus establishes infection, enters the host, spreads within it, targets particular organs, produces disease, and is shed.

→ See Viral virulence and host factors for why the same virus produces a spectrum of disease across a population, the viral determinants of virulence set against the host factors that govern resistance.

→ See Patterns of infection for the longitudinal patterns infection settles into, transient and persistent, latent and chronic, and the markers that distinguish them.

→ See Viral immunopathology for the diseases caused by the immune response itself, from the cytokine storm and antibody-dependent enhancement to post-infectious autoimmunity.

Key terms

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

Core distinctions:

Term	Definition
Infection	The entry and replication of a virus in a host, which may or may not produce symptoms.
Disease	The clinical manifestation of tissue damage or physiological disruption, caused by the virus or by the host’s response to it.
Pathogenicity	The qualitative capacity of a virus to cause disease in a susceptible host: a virus is pathogenic or it is not.
Virulence	The quantitative degree of disease a virus causes, used to compare a virulent strain with an attenuated, avirulent one.

Tropism and entry:

Term	Definition
Susceptible	A cell bearing the functional surface receptor a virus needs to attach and enter.
Permissive	A cell providing the internal machinery and environment for a virus to complete its replication cycle.
Tropism	The predilection of a virus for a particular cell type, tissue, or host species, set by both susceptibility and permissiveness.

Kinetics and spread:

Term	Definition
Incubation period	The interval from exposure to the onset of clinical signs and symptoms.
Latent period	The interval from infection to the onset of viral shedding, when the host is infected but not yet infectious. Distinct from viral latency, which is a dormant state of infection rather than a time interval.
Viral titre	The concentration of infectious virus in a sample.
Primary viraemia	The first entry of virus into the blood, disseminating it from the entry site to secondary target organs.
Secondary viraemia	A later, higher wave of virus in the blood, following massive replication in those secondary organs.

Course and recurrence:

Term	Definition
Primary infection	A host’s first encounter with a given virus, as distinct from any later reactivation or reinfection.
Reinfection	A fresh infection by the same or a related virus acquired anew from outside, as opposed to reactivation of a virus already latent in the host.
Transient infection	An infection that is cleared once the immune response develops, usually leaving immunity.
Persistent infection	An infection that is not cleared, continuing for months, years, or life, whether latent or chronically active.
Viral latency	A persistent infection in which the genome lies dormant without producing infectious virions, retaining the potential to reactivate. Distinct from the latent period, an epidemiological time interval.
Reactivation	The return of a truly latent virus, carried as a dormant, non-replicating genome, to active replication.
Chronic infection	A persistent infection with continuing viral replication and often progressive injury, as distinct from the dormancy of latency.
Recrudescence	The clinical flare of a persistent, low-level (smouldering) infection that rises to clinical relevance, as distinct from reactivation, which proceeds from true latency.
Recurrence	A repeat episode of clinical disease, whether from reactivation of a latent virus, recrudescence of a smouldering one, or reinfection from outside.

Cellular damage and pathology:

Term	Definition
Cytopathic effect	The visible morphological change or damage in a cell caused by viral replication, such as rounding, detachment, or lysis.
Cytocidal (cytolytic) and non-cytocidal	Whether a virus kills the cell it infects; a non-cytocidal virus produces progeny without killing the cell, which underlies much persistence and immunopathology.
Syncytium	A multinucleated giant cell formed when an infected cell fuses with its neighbours, driven by viral fusion proteins.
Inclusion bodies	Histologically visible aggregates of viral components within the nucleus or cytoplasm of an infected cell.
Immunopathology	Tissue damage driven by an excessive or dysregulated host immune response rather than by direct viral injury, as in a cytokine storm.

Immune evasion and host interaction:

Term	Definition
Antibody-dependent enhancement	Worsening of infection when sub-neutralising antibody ferries virus into immune cells through their Fc receptors, as in dengue.
Molecular mimicry	Resemblance between viral and host antigens that can provoke a cross-reactive, autoimmune response.
Quasispecies	The swarm of genetically related variants that make up a virus population within a host, generated by replication errors and serving as the substrate for immune and drug escape.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Pathogenesis of Virus Infections; Patterns of Infection. In: Fenner and White’s Medical Virology, 5th edition, Chapters 7 and 8. Academic Press / Elsevier; 2017. The foundational account of the course of infection, the determinants of outcome, and the patterns of transient and persistent infection.
• Morrison TE, Heise MT. Pathogenesis of Viral Infection. In: Fields Virology, 7th edition, Volume 4, Chapter 8. Wolters Kluwer; 2023. The current reference for the integrated virus-host framework of pathogenesis, the sequential stages of infection, and the determinants of virulence and tropism.