Viral Transmission

A virus cannot replicate outside a living cell, so its survival in nature depends on passing from one host to the next before the current infection is cleared or the host dies. Transmission is the event that every epidemiological measure ultimately counts, and its frequency and dynamics are quantified separately. Each virus uses a defined route or small set of routes, set by three things: where in the body the virus is shed, how stable the virion is outside the host, and which tissues the virus infects.

Transmission is described through two complementary frameworks, and it helps to hold both, because the literature uses each. The first is generational, concerned with lineage: horizontal transmission passes a virus between individuals in a population, while vertical transmission passes it from parent to offspring, in utero, around birth, or through breast milk, a route whose dependence on host survival tends to select for lower virulence. The second is mechanistic, concerned with the physical pathway: direct transmission needs immediate contact between an infected and a susceptible host, such as skin or sexual contact, a bite, or short-range respiratory droplets, whereas indirect transmission bridges a gap through an intermediary, an inanimate fomite, a contaminated vehicle such as food or water, or a living vector. The two axes are orthogonal and combine: vertical transmission is almost always direct, while horizontal transmission runs through both direct and indirect pathways.

Viral shedding and the portals of exit and entry

Transmission begins with shedding, the release of infectious virus from an infected host, and for most viruses the site of shedding mirrors the site of entry: a virus that enters through the respiratory tract is generally shed from it, and one that enters through the gut is shed in faeces. Localised infections shed from the single surface they infect, whereas generalised infections can shed from several sites at once, so a systemic virus such as cytomegalovirus appears in saliva, urine, semen, cervical secretions and milk.

The amount of virus shed decides whether a route is efficient. Very low concentrations may be irrelevant unless a large volume is transferred, whereas some viruses reach such high titres that under a microlitre of fluid can transmit infection. Individuals vary too, and a minority of hosts who shed unusually large amounts, sometimes called super-shedders, contribute disproportionately to spread. Not every site of replication leads to shedding: virus produced in the brain or in an internal organ with no route to a body surface reaches a dead end and is not transmitted, even when the infection there is severe or lethal.

Routes of transmission

The major horizontal routes are defined by how the virus crosses the epithelial lining of the body. Each pairs a portal of exit with a portal of entry, and the stability of the virion outside the host largely sets how far and how long the route can operate.

Route	Mechanism	Portal of exit to entry	Representative viruses
Respiratory	Aerosols and droplets from coughing, sneezing and talking	Respiratory tract to respiratory tract	Influenza, measles, rhinovirus, SARS-CoV-2
Faecal-oral	Shedding in faeces contaminating food, water and hands	Gut to mouth	Enteroviruses, hepatitis A, rotavirus, norovirus
Contact (cutaneous)	Direct contact with skin lesions or through abraded skin	Skin to skin	Herpes simplex, papillomaviruses, molluscum, poxviruses
Sexual (genitourinary)	Mucosal contact with genital secretions	Genital mucosa to genital mucosa	HIV, hepatitis B, herpes simplex type 2, HPV
Blood-borne	Inoculation of infected blood or blood products	Blood to blood	HIV, hepatitis B, hepatitis C, HTLV
Ophthalmic	Contaminated fingers, water or instruments	Conjunctiva to conjunctiva	Adenoviruses, enterovirus 70
Milk-borne	Virus excreted in breast milk	Milk to gut	Cytomegalovirus, HIV-1, HTLV-1
Vertical	Germline, transplacental or perinatal transfer	Mother to fetus or newborn	Rubella, cytomegalovirus, HIV, hepatitis B

The respiratory route is the most efficient for rapid, wide spread, because shedding into aerosols during coughing, sneezing and talking reaches many contacts quickly. Historically the spread was divided by particle size: large droplets, over a few micrometres across, fall within about a metre and need close contact, while small droplet nuclei, under five micrometres, evaporate and can travel further and settle deeper in the airway, with contaminated surfaces (fomites) a third component. This clean droplet-versus-airborne split is the classical teaching; the reappraisal of fine-aerosol spread that followed SARS-CoV-2 has since blurred it into a continuum, so the size cut-off is best read as an approximation rather than a sharp boundary. Enveloped respiratory viruses are labile and do not survive long outside the body, which is why this route depends on relatively close or repeated contact.

The faecal-oral route depends on hardier, usually non-enveloped virions that survive in water and on surfaces for days to weeks, shed in large numbers in faeces and transmitted through contaminated food, water and hands. Contact transmission requires direct skin-to-skin contact for viruses that infect the skin or that are present in vesicle fluid, since unbroken skin is otherwise an effective barrier. Sexual transmission moves viruses kept moist in genital secretions across mucosal surfaces, and is more efficient where the mucosa is breached by ulcerative infection. Blood-borne transmission, once dominated by transfusion and now largely by shared injecting equipment, carries the viruses that maintain a persistent viraemia. The ophthalmic route introduces virus to the conjunctiva from fingers, water or instruments, and the milk-borne route delivers virus in breast milk to the infant gut.

Vertical transmission passes a virus from mother to offspring by one of three mechanisms: integration into the germline, transplacental spread during pregnancy, or perinatal and postnatal spread during delivery and breastfeeding. Its consequences range from fetal loss and congenital malformation, as in congenital rubella and cytomegalovirus, to a silent infection that seeds a new generation, as for hepatitis B and HIV, which then transmit onward for decades.

Arthropod-borne transmission

A large group of viruses, the arboviruses, are transmitted by the bite of a blood-feeding arthropod. The defining feature is that the virus replicates within the vector before it can be transmitted, so after the insect takes an infected blood meal there is an extrinsic incubation period, the days to weeks during which the virus multiplies and reaches the salivary glands before the vector becomes infectious; its length shortens at higher ambient temperature. This biological transmission, in which the vector is an essential host, is distinct from the much rarer mechanical transmission, in which a virus is carried passively on contaminated mouthparts without replicating. In some vectors the virus also passes from one generation to the next through the egg, which helps it survive between transmission seasons. The ecological cycles that maintain these viruses, together with their vectors and reservoirs, define the arthropod-borne viruses as a group.

Nosocomial and iatrogenic transmission

Transmission can also occur within healthcare. Nosocomial transmission is acquisition of a virus in a hospital or clinic, favoured because infectious and susceptible patients are concentrated together and because invasive procedures breach normal barriers; respiratory syncytial virus, influenza and varicella spread this way on wards, and the blood-borne viruses through needlestick injury and contaminated equipment. Iatrogenic transmission is spread by a medical procedure or practitioner, the extreme historical example being the 1976 Ebola outbreak in Zaire, amplified by the reuse of unsterilised needles. Both are preventable by the standard measures of infection prevention and control.

The dynamics of spread

How fast a virus spreads through a population is captured by the basic reproduction number, R₀. It is not a single fixed quantity but the product of three components: how efficiently the virus is transmitted at each contact, how many contacts an infected person makes per unit time, and how long the person remains infectious; for an arbovirus the density and competence of the vector enter as well. Anything that lowers one of these components lowers R₀, which is why the same virus can have a different R₀ in different populations. Once immunity accumulates the relevant quantity is the effective reproduction number, R, equal to R₀ times the susceptible fraction, and control works by holding R below 1.

The tempo of spread is set by the generation time, the average interval between one infection and the infections it causes; the serial interval, the gap between symptom onset in successive cases, is its observable proxy. Two other intervals matter for control. The incubation period is the time from infection to the onset of symptoms, while the latent period is the time from infection to the onset of infectiousness. When the latent period is shorter than the incubation period, a person becomes infectious before feeling ill, and transmission occurs pre-symptomatically, which is the case for measles and chickenpox and which blunts any control measure that relies on detecting symptoms. The proportion of transmission occurring before symptoms, sometimes written as theta, is therefore a key parameter: SARS in 2003 had a low pre-symptomatic fraction and a generation time of about a week, which is precisely why quarantine and isolation contained it.

Averages conceal a further feature. Transmission is often overdispersed, with a small number of individuals or events causing most secondary cases, the superspreading pattern seen with SARS and later with SARS-CoV-2. Overdispersion makes an epidemic’s early course erratic, since a chain may fade out by chance or explode from a single event, and it means control focused on high-transmission settings can be disproportionately effective. Finally, the classical models assume a uniformly mixing population, an assumption long-distance air travel has broken: a single infected traveller can seed a new susceptible population far away, so a modern epidemic is a set of local outbreaks linked by a few long-range movements, as when SARS reached three continents within days of the index case arriving in Hong Kong.

Determinants of transmissibility

Whether a contact results in transmission depends on the agent, the host and the environment together. The agent contributes its infectivity, the probability of infecting a susceptible host on exposure, often summarised as the dose infecting half of those exposed; its pathogenicity, the probability that infection causes disease; and its virulence, the probability of severe disease, read from the case-fatality rate. The host contributes susceptibility, which turns on prior immunity from infection or vaccination, and on age, pregnancy and immunosuppression, all of which raise the chance and the consequences of infection. The environment contributes the physical stability of the virion in prevailing temperature and humidity, and the social conditions, above all crowding and contact patterns, that determine how often infectious and susceptible people meet. The seasonal and population-level expression of these determinants shapes how and where a virus endures.

South African context

South Africa’s transmission patterns are shaped by a high burden of the persistent, vertically and sexually transmitted viruses, HIV and hepatitis B above all, whose control depends on interrupting mother-to-child and sexual transmission through antenatal screening, infant immunisation and treatment as prevention. High population density and a large tuberculosis burden amplify respiratory transmission in congregate settings, and the safety of the blood supply against the blood-borne viruses rests on comprehensive donor screening by the South African National Blood Service. These interventions are delivered through the relevant clinical and prevention programmes, and the same transmission routes seen everywhere, weighted differently by local conditions, drive South Africa’s epidemiology.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Epidemiology of Viral Infections. In: Fenner and White’s Medical Virology, 5th edition, Chapter 13. Academic Press / Elsevier; 2017. The foundational backbone for viral shedding, the routes of transmission, and the parameters governing the dynamics of spread.
• van Seventer JM, Hochberg NS. Principles of Infectious Diseases: Transmission, Diagnosis, Prevention, and Control. In: International Encyclopedia of Public Health, 2nd edition, Volume 6. Elsevier; 2017. p. 22–39. A public-health framework source for the modes of transmission, the reproduction number and the agent, host and environmental determinants of transmissibility.
• Wang CC, Prather KA, Sznitman J, Jimenez JL, Lakdawala SS, Tufekci Z, Marr LC. Airborne transmission of respiratory viruses. Science. 2021;373(6558):eabd9149. The authority for the reframing of the classical droplet-versus-airborne dichotomy as an aerosol continuum, underpinning the treatment of the respiratory route here.