Patterns of Infection

The many ways a virus can move through a host resolve, at the level of the whole infection, into a small number of recurring patterns. A virus is either cleared after a brief encounter or it stays for the life of the host; its replication is either confined to the surface where it entered or it spreads through the body; and the infection it causes is either silent or overt. These distinctions sort every human virus infection into a few categories, and the category an infection belongs to is what determines its natural history, the markers used to diagnose it, and the strategy needed to treat or prevent it.

Productive, abortive, and latent infection

Before an infection can follow any pattern, a virus must encounter a cell it can use, and two independent properties of that cell decide what happens. A cell is susceptible if it carries the functional receptor a virus needs to attach and enter, and permissive if it also provides the internal machinery, the transcription factors and cofactors and the right cellular environment, to complete the replication cycle. The two are separate: a susceptible cell may be permissive or not, and only a cell that is both supports a productive infection, one that yields new infectious virus.

When a virus enters a cell that cannot support its full cycle, the infection is abortive: the genome enters but no progeny are made. Between these lies latency, in which a virus enters a cell, makes no infectious progeny, but retains its genome and the capacity to resume replication later, on reactivation. Latency is not a failed infection but a distinct programme of gene expression, and it is the foundation of the persistent patterns described below. Whether a given cell is permissive can itself change with the cell’s state, so a virus may replicate productively in one tissue and lie latent in another.

The patterns of infection

Two axes organise the patterns. The first is duration: a transient infection is cleared once the immune response develops, while a persistent infection continues for months, years, or life. The second is extent: a localised infection stays at the epithelial surface where the virus entered, while a systemic infection spreads through the body, usually by way of the blood. Crossing the two axes gives four patterns, and any given virus can be placed in one of them.

Pattern	Localised (confined to the entry surface)	Systemic (spreads through the body)
Transient	Common cold (rhinoviruses), influenza, rotavirus gastroenteritis	Measles, varicella, mumps, poliomyelitis
Persistent	Papillomavirus warts	Herpesvirus latency; chronic hepatitis B and C; HIV

Overlaid on this is the distinction between an asymptomatic, or subclinical, infection and a symptomatic one. The proportion that is symptomatic varies enormously by virus: it approaches the whole of those infected for measles, but is under five per cent for cytomegalovirus in the healthy, and a minority at the initial infection for HIV and hepatitis C, even though almost all of those untreated HIV infections eventually cause disease. The two patterns of transient infection also differ characteristically in their incubation period. A transient localised infection, in which disease arises at the site of entry, has a short incubation of about two to five days. A transient systemic infection, in which the virus must replicate locally, seed the blood, replicate again, and reach a distant target organ, has a longer incubation of around ten to forty days, the interval reflecting the time the sequential stages take to play out.

Transient infections

Most human virus infections are transient. The virus replicates, the immune response develops, and the agent is eliminated, leaving immunity that may be lifelong or short-lived. The mechanics of how these infections spread, and why a localised infection can still be severe, are set out in the course of a viral infection. The localised infections are the common respiratory and gastrointestinal illnesses, their short incubation reflecting disease at the site of entry. The systemic infections include many of the classic childhood exanthems, which spread through the body in a defined sequence of steps before producing their characteristic disease, and which typically confer durable immunity, the property vaccination exploits.

Persistent infections

A growing number of viruses are not cleared but persist, and persistence matters out of proportion to the number of agents involved. A persistent carrier is a continuing source of virus, sustaining it in a population even where transmission is inefficient; a persistent genome may reactivate to cause acute disease; chronic infection may produce slowly progressive organ damage or cancer; persistence can confound the diagnosis of an unrelated illness; and a latent genome, not replicating, is invisible to antivirals that act on replication, which is why persistent infections are so hard to cure.

Persistence requires two conditions together: the immune system must fail to eliminate the virus, and, for any virus that would otherwise kill its host cell, replication must be restrained. The molecular strategies that achieve this, restricted gene expression and latency, sequestration in sanctuary sites, antigenic variation, virus-induced immunosuppression, and tolerance, are the subject of viral immune evasion and are not re-derived here; the concern of this section is the clinical patterns they produce. Those patterns fall into a persistent localised form and three persistent systemic forms.

Category	Virus	Site of persistence	Associated disease
Persistent localised	Papillomaviruses	Skin and mucosal epithelium	Warts; cervical and other cancers
Acute, with late complications	Measles virus	Central nervous system	Subacute sclerosing panencephalitis
	JC polyomavirus	Kidney, central nervous system	Progressive multifocal leukoencephalopathy
Latent, with reactivation	Herpes simplex viruses	Sensory ganglia	Recurrent oral or genital herpes
	Varicella-zoster virus	Sensory ganglia	Herpes zoster (shingles)
	Cytomegalovirus	Myeloid progenitor cells, glands	Pneumonitis, retinitis, colitis
	Epstein-Barr virus	B lymphocytes	Lymphomas, nasopharyngeal carcinoma
Chronic, with ongoing replication	Hepatitis B and C viruses	Liver	Cirrhosis, hepatocellular carcinoma
	HIV	CD4 T cells, macrophages	AIDS
	HTLV-1	T lymphocytes	Adult T-cell leukaemia, tropical spastic paraparesis

Persistent localised infection

The papillomaviruses are the model. Infection is established in the basal layer of an epithelium, where viral gene expression is restricted, and complete replication with assembly of infectious virus occurs only as the infected cell differentiates and moves toward the surface, the layers least accessible to immune surveillance. The result is a wart, usually adjacent to the site of entry, persisting for months. That the immune system retains some control is shown by the way warts become more numerous under immunosuppression and can regress together when control is restored, and the oncogenic genotypes, through a sustained drive to proliferate, can progress to cervical and other carcinomas.

Acute infection with late complications

A few viruses persist silently after an apparently complete recovery and cause a rare, delayed, and usually fatal disease. Subacute sclerosing panencephalitis is the paradigm: a progressive and lethal encephalitis arising years after measles, in which defective virus persists in neurons at very low level. Progressive multifocal leukoencephalopathy is the reactivation of latent JC polyomavirus, near-universal and silent in the healthy, which destroys the oligodendrocytes of the brain only when cellular immunity fails, as in advanced HIV or under immunosuppressive therapy.

Latent infection with reactivation

Latency with reactivation is the defining behaviour of the eight human herpesviruses. Each establishes lifelong carriage after the primary infection, with the genome maintained in a long-lived cell and a restricted set of genes expressed, and each can reactivate to resume full replication, sometimes silently and sometimes as recurrent disease. Profound immunosuppression, in transplantation or advanced HIV, can make reactivation severe or fatal. The site of latency divides them: the alphaherpesviruses persist in non-dividing neurons, while several of the others persist in dividing lymphoid or myeloid cells, which requires the genome to be copied each time the host cell divides.

Virus	Site of latency	Reactivation disease
Herpes simplex virus 1 and 2	Sensory ganglion neurons	Recurrent cold sores; genital herpes
Varicella-zoster virus	Sensory ganglion neurons	Herpes zoster (shingles)
Cytomegalovirus	CD34 myeloid progenitor cells	Pneumonitis, retinitis, colitis in the immunocompromised
Epstein-Barr virus	Memory B lymphocytes	Lymphoproliferative disease and lymphomas

The mechanism of latency and reactivation is treated under viral immune evasion; the clinically important point here is that a latent virus is present but not replicating, so it neither causes disease nor responds to antivirals between episodes, and the trigger for its reappearance is most often a lapse in the immune control that was holding it silent.

Chronic infection with ongoing replication

In chronic active infection the virus is never cleared and continues to replicate, demonstrably and often abundantly, while disease is absent, slowly progressive, or delayed. Hepatitis B, hepatitis C, and HIV are the agents of greatest public health weight. Continuous replication makes these infections a moving target: the within-host virus population exists as a quasispecies, a swarm of related variants generated by copying errors, from which immune and drug pressure continually select escape mutants, which is a central reason a sterilising immune response or a curative vaccine has been so hard to achieve for HIV and hepatitis C. The disease they cause is largely a function of time and of the host response, the progressive fibrosis and the hepatocellular carcinoma of chronic viral hepatitis, the immune collapse of untreated HIV.

The course of a persistent infection is not static, and several distinct developments can change it, for better or worse.

Process	Example	Result
Gradual recovery of immunocompetence	Hepatitis B	Elimination, with or without hepatitis flares; immune restoration disease
Emergence of escape mutants	Equine infectious anaemia	Flares of viraemia, with or without disease
Immunosuppression	HIV	Increased replication; opportunistic infections and malignancies
Progressive tissue damage	Hepatitis C, HIV	Organ failure
Virus-directed cell transformation	HTLV-1, papillomaviruses	Development of cancer
Cell damage with increased proliferation and mutation	Hepatitis B and C	Development of cancer

Distinguishing the patterns in the laboratory

Identifying which pattern an infection has taken is one of the routine tasks of diagnostic virology, and it rests on combining serological and molecular markers. A rising titre of specific immunoglobulin M (IgM), the first antibody class produced, marks a recent or acute infection, while immunoglobulin G (IgG) without IgM indicates past infection and existing immunity; testing paired samples taken weeks apart, or measuring the avidity of the IgG, refines the timing. Detection of viral antigen or nucleic acid shows that virus is present, and its quantity, the viral load, separates the patterns: a latent infection between episodes yields little or no detectable antigen or circulating genome despite lifelong carriage, whereas a chronic active infection yields a persistently detectable, often high, viral load. Hepatitis B is the worked example, its surface antigen, e antigen, core antibodies, and viral DNA combining into serological profiles that distinguish acute infection, the chronic carrier state, resolved past infection, and immunity from vaccination. Reading these markers against the patterns is what lets the laboratory tell a resolving acute infection from established latency and from ongoing chronic disease.

Clinical and therapeutic significance

The pattern an infection follows sets the strategy for managing it. A transient infection is treated, if at all, in its brief window of replication, and is most powerfully addressed by the vaccination that pre-empts it. A latent infection cannot be eradicated by antivirals that act only on replication, so management aims at suppressing or pre-empting reactivation, particularly around planned immunosuppression, and at treating the reactivation episodes when they occur. A chronic active infection calls for long-term suppressive therapy, monitored by viral load, to hold replication down and arrest the progressive injury, even where cure is not achievable. And because reactivation and progression both track the failure of immune control, the immune status of the host is, across all the persistent patterns, the single most important variable in anticipating what a persistent virus will do.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Patterns of Infection. In: Fenner and White’s Medical Virology, 5th edition, Chapter 8. Academic Press / Elsevier; 2017. The principal source for the transient and persistent patterns of infection, the persistent localised and systemic categories, and the pathological changes that develop during persistent infection (Tables 8.5 and 8.8).
• 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 susceptible-versus-permissive distinction, the productive, abortive, and latent infection states, and the role of viral quasispecies in chronic infection.