virulent

Virulent is an adjective used in biology to describe pathogens that can cause disease, often with severe effects. It also refers to the degree of harmfulness, or virulence, of a microorganism. The word has extended to describe highly aggressive or intense phenomena in non-biological contexts, but its primary medical meaning relates to a disease-causing capacity. Etymology: from Latin virulentus meaning poisonous or full of venom, derived from virus meaning poison.

In microbiology, virulence is determined by a combination of host, microbe, and environmental factors. Microbes possess virulence factors that promote colonization, invasion, immune evasion, and toxin production. Examples include adhesins that help attachment to host tissues, capsules that resist phagocytosis, toxins such as those produced by diphtheria or botulinum, siderophores that acquire iron, and secretion systems that inject effectors into host cells. The observed harm depends on host immunity, the route of exposure, and the microbial load.

Virulence is often discussed alongside pathogenicity. Virulence describes the degree of harm caused by a pathogen once disease develops, while pathogenicity refers to the overall ability to cause disease. It is commonly measured in experimental models by estimates such as the LD50 (the dose needed to kill 50% of hosts) or the ID50 (the dose needed to establish infection in 50%). Attenuation reduces virulence, a principle used in vaccines and in the development of safer laboratory strains. Virulence can evolve through mutation, loss or gain of virulence genes, and horizontal gene transfer, and it can vary across different host species and environments.