Viral Virulence and Host Factors

Infection with one virus produces a spectrum of outcomes across the people it infects, from no symptoms at all to fatal disease. That spread is not random. It is the integrated product of two sets of factors that meet at every stage of the infection: how virulent the virus is, and how resistant the host is. A highly virulent virus in a susceptible host produces severe disease; the same virus in a resistant host, or an attenuated virus in a susceptible one, may produce none. Understanding which viral and host factors govern this balance is what allows the clinician to anticipate who will do badly, and the virologist to judge which emerging viruses are dangerous.

Pathogenicity and virulence

Two terms are easily confused. Pathogenicity is the qualitative capacity of a virus to cause disease at all: a virus is either pathogenic in a given host or it is not. Virulence is the quantitative measure of how much disease it causes, the degree of pathology, which is why one speaks of a highly virulent strain against an attenuated, avirulent one. Because virulence is a matter of degree, it is always relative, and meaningfully compared only between related viruses in a defined host: it makes sense to say one influenza strain is more virulent than another, but not to rank an influenza virus against a herpesvirus, whose disease takes an entirely different form.

Virulence is also distinct from transmissibility, and the two need not move together. The 2009 pandemic influenza H1N1 virus was highly transmissible but of low virulence, spreading worldwide while causing mostly mild disease. The highly pathogenic avian influenza H5N1, by contrast, is poorly transmissible between humans yet kills a large proportion of those it does infect. A virus that is both highly transmissible and highly virulent is the most dangerous combination, and predicting where a new virus sits on these two axes is central to assessing its threat. A further property, invasiveness, describes how efficiently a virus reaches and damages a target tissue: two viruses of similar intrinsic virulence can differ greatly in whether they ever reach the organ where they would do harm.

Virulence is quantified in the laboratory by titrating a virus in groups of animals and recording the dose that kills or sickens half of them, the median lethal dose (LD50) or median infective dose (ID50), supplemented by measures such as the time to death or the degree of weight loss. Human virulence cannot be measured this way and is instead inferred from the severity observed in natural outbreaks, an inference that is harder for chronic infections, where a more aggressive course may be the cause of a worsening illness or merely its consequence.

Viral determinants of virulence

No single “virulence gene” usually accounts for how dangerous a virus is. Virulence is a cooperative property of many genes, a constellation, and attempts to pin it to one determinant are usually inconclusive. The viral genes that contribute fall into four broad categories.

Category of viral gene	Contribution to virulence
Replication efficiency	How rapidly and abundantly the virus replicates, setting the pace of the race against host defences
Evasion of host defences	Genes that blunt interferon, antigen presentation, and other immune mechanisms
Tropism, spread, and transmissibility	Genes that determine which tissues are reached and how readily the virus disseminates and is shed
Directly toxic products	The rare viral proteins that injure the host directly

The first category, replication, is illustrated by the attenuated oral polio vaccine strains, which differ from virulent poliovirus by only a few nucleotides; the key changes lie in the internal ribosome entry site within the 5′ untranslated region (5′-UTR) of the genome and reduce the efficiency with which the viral proteins are translated, weakening replication enough to abolish neurovirulence while preserving immunogenicity.

The third category, tropism and spread, is illustrated by influenza. The haemagglutinin must be cleaved by a host protease to become active; avirulent strains carry a monobasic cleavage site recognised only by proteases confined to the respiratory or gastrointestinal tract, whereas highly pathogenic avian strains carry a polybasic site cleaved by furin, an enzyme present in tissues throughout the body, allowing systemic spread.

Influenza tropism also turns on the sialic-acid linkage its haemagglutinin prefers: human-adapted viruses bind the alpha-2,6-linked sialic acid abundant in the upper airway, favouring transmission, while avian viruses bind the alpha-2,3-linked form of the lower respiratory tract, favouring severe pneumonia but poor spread.

Directly toxic viral products are rare, because a successful virus keeps its host cell working, but the rotavirus non-structural protein NSP4 is a clear example, acting as an enterotoxin that drives the secretory diarrhoea of rotavirus infection.

The second category, evasion of host defences, is a major contributor to virulence and is treated in detail under viral immune evasion; the hepatitis C virus protease that cleaves the interferon adaptor MAVS, and the many viral proteins that downregulate the major histocompatibility complex, are virulence determinants in this sense. Not every virulence determinant is a protein. Non-coding elements of viral genomes, among them regulatory RNA structures in flaviviruses and alphaviruses, also modulate virulence by shaping the interferon response, a reminder that the genetic basis of virulence extends beyond the coding sequence.

Host determinants of outcome

The host contributes as much to the outcome as the virus. The same virus is more or less virulent depending on who it infects, and an otherwise avirulent virus, or even a live-attenuated vaccine strain, can cause severe disease in a profoundly immunocompromised person.

Host factor	Influence on outcome	Example
Genetics	Inherited variation in immune and receptor genes alters susceptibility	TLR3 defects and herpes simplex encephalitis; CCR5 mutation and HIV resistance
Age	Outcome often worst at the extremes, but the pattern varies by virus	Severe respiratory syncytial virus in infants; severe influenza in the elderly
Sex	Some infections run a more severe course in one sex	Male-biased severe COVID-19
Nutrition	Malnutrition weakens barriers and immune responses	Severe, high-mortality measles in malnourished children
Pregnancy	Some infections are markedly more severe; others reactivate	Severe hepatitis E; reactivation of herpesviruses
Microbiome	Resident bacteria tune immunity and can help or hinder a virus	Gut bacteria promoting enteric virus infection
Co-infection	A concurrent infection can worsen or, occasionally, protect	Tuberculosis accelerating HIV progression
Immune status	Immunodeficiency allows severe or disseminated disease	Life-threatening cytomegalovirus in transplant recipients

Host genetics gives the clearest demonstrations, because a single defect can unmask the role of one pathway. Inborn errors of interferon immunity, in genes such as TLR3 (Toll-like receptor 3) and UNC-93B, predispose to herpes simplex encephalitis, and loss-of-function variants in the type I interferon genes, together with autoantibodies against type I interferon, account for a substantial share of life-threatening COVID-19. A mutation in the HIV co-receptor CCR5 confers strong resistance to HIV, variation in the human leukocyte antigen (HLA) class I genes shapes the control of HIV by cytotoxic T cells, and a polymorphism in the interferon-lambda gene predicts clearance of hepatitis C. Most susceptibility, however, is polygenic, the summed effect of variation across many genes rather than any single one.

Age is the host factor encountered most often at the bedside, and its effect is not simply that the very young and very old fare worst, though they often do. The pattern is virus-specific.

Effect of age	Examples
More severe in infancy than later childhood	Rotavirus, respiratory syncytial virus, herpesviruses, coxsackieviruses
More severe in older children and adults	Poliomyelitis, hepatitis A, measles, Epstein-Barr virus, varicella, mumps
Age-dependent chronicity	Hepatitis B: persistence in over 90 per cent of infections acquired in infancy, but only 5 to 10 per cent of those acquired in adulthood
Faster progression in middle age and beyond	HIV, hepatitis C
Greater mortality in the elderly	Influenza, and many acute infections, through declining immunity and comorbidity

The age effect reflects several mechanisms at once: the immaturity of the infant immune system, the immunosenescence of old age, anatomical factors such as the narrow airways that make respiratory syncytial virus so dangerous in infants, and occasionally a developmental susceptibility of the target cell itself. It is not always the frail who suffer most. The 1918 influenza pandemic killed an unusual excess of healthy young adults, in whom a vigorous inflammatory response is thought to have contributed to the lethality, and mumps orchitis is rare before puberty but common after it. The remaining host factors, sex, nutrition, pregnancy, the microbiome, co-infection, and fever, each shift the balance in their own way: fever, for instance, is generally protective, and its suppression can worsen the course of an infection.

The virus-host balance and viral evolution

Because outcome is the joint product of viral virulence and host resistance, neither side is fixed: virus and host evolve in response to one another. The clearest demonstration came from the release of myxoma virus to control European rabbits in Australia, which began at over ninety-nine per cent mortality but within a few years settled toward an intermediate virulence, as the rabbit population was selected for resistance and the virus itself attenuated. The finding reshaped how virologists think about virulence. It showed that a virus does not evolve toward ever-greater lethality, because a host that dies too quickly transmits poorly, nor toward harmlessness, because too little replication transmits poorly either; virulence is selected toward whatever level optimises transmission. Virulence is therefore a transmissible trait shaped by natural selection, not an intrinsic constant, and host and virus co-evolve.

The same logic explains why newly emerging viruses are so often dangerous. A virus long established in its natural host is typically of low virulence, having been selected toward coexistence, whereas the same virus on jumping to a new host species, one with which it has no evolutionary history, may cause severe disease. Herpes B virus is benign in its macaque reservoir but lethal in the humans it occasionally infects. This principle, low virulence in the adapted host and high virulence on a species jump, is one of the foundations of understanding zoonotic emergence, and it is why surveillance of animal viruses with the potential to cross into humans matters so much.

Clinical and therapeutic significance

The practical value of these determinants is in stratifying risk and directing care. Knowing that the host factors carry so much of the outcome is what identifies the patients in whom an ordinary virus becomes an emergency: the neonate, the pregnant woman, the elderly, the malnourished, and above all the immunocompromised, in whom even attenuated vaccine strains can be dangerous and in whom prophylaxis and early antiviral treatment are most justified. Knowing the viral determinants explains why a vaccine strain is safe, why a polybasic cleavage site in an avian influenza isolate is an alarm signal, and why genetic surveillance of circulating viruses can flag a rise in virulence before it is clinically obvious. Reading virulence as a property of a virus in a particular host, finally, is what keeps risk assessment honest: the question is never simply how dangerous a virus is, but how dangerous it is to this patient, or to this population.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Pathogenesis of Virus Infections. In: Fenner and White’s Medical Virology, 5th edition, Chapter 7. Academic Press / Elsevier; 2017. The principal source for the definitions of pathogenicity and virulence, the genetic determinants of viral virulence, and the host factors affecting the severity of infection (Table 7.10).
• 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 determination of outcome, the discipline of comparing virulence only between related viruses, the host-factor framework, and the evolution of virulence toward optimal transmission.