transmissionvirulence

Transmission virulence, sometimes called the virulence–transmission trade-off, is a concept in evolutionary epidemiology describing how a pathogen's virulence and its transmission opportunities interact to shape its evolution. In many systems, higher virulence can enhance within-host replication and shedding, increasing transmission, but it can also shorten host lifespan or reduce host contact, lowering transmission opportunities. The balance between these opposing effects can select for intermediate levels of virulence.

The outcome depends on transmission mode, duration of infection, host behavior, and medical or public health interventions. For directly transmitted pathogens, harming the host quickly can limit transmission, whereas vector-borne pathogens may experience a weaker link between host virulence and transmission since vectors can spread the pathogen even when the host is partly compromised. Host immunity, coinfection, and environmental conditions also influence the trade-off.

Empirical patterns vary. Many studies are broadly consistent with a trade-off, but not universal; some pathogens show strong coupling between virulence and transmission, others show weak or context-dependent relationships due to external drivers such as treatment, vaccination, or seasonal factors. Evolutionary models typically represent virulence as a parameter that affects transmission rate and infection duration, and seek the level of virulence that maximizes pathogen fitness, often approximated by R0.

Public health implications include how interventions can alter selective pressures on virulence and affect outbreak dynamics. See also virulence, transmission, the trade-off hypothesis, and pathogen evolution.