Virulence

Virulence refers to the degree of damage or disease severity caused by a pathogen in a host. It is a measure of pathogenicity, but it emphasizes how severely an infection affects the host rather than simply the capacity to cause disease. Virulence is not fixed for a microbe; it depends on interactions among the pathogen, the host, and the environment. Consequently, virulence can vary with host factors such as immune status, age, and genetics, as well as with details of the infection such as dose and route of entry.

Virulence is driven by virulence factors, which are molecules or structures that enable a microbe to invade, replicate, evade the immune system, or damage host tissues. Common virulence factors include toxins (exotoxins and endotoxins), enzymes that facilitate tissue invasion, capsules and surface proteins that resist phagocytosis, adhesion factors that promote colonization, and systems for acquiring nutrients like iron. The expression of virulence genes is often regulated and context-dependent, responding to environmental signals and host defenses.

Determinants of virulence encompass both pathogen traits and host conditions. The infectious dose, route of entry, and tissue tropism influence disease outcome. Host factors such as immunity, microbiota composition, age, and underlying health conditions can amplify or mitigate virulence. Pathogens may also adapt virulence levels through evolution, particularly in relation to transmission potential.

In epidemiology and public health, virulence is considered alongside transmissibility. The transmission–virulence trade-off suggests that higher virulence can reduce host mobility and survival, potentially lowering transmission, while moderate virulence may maximize spread. Understanding virulence informs vaccine design, therapeutics, and risk assessment.