infectiousness

Infectiousness is the capacity of a pathogen to be transmitted from an infected host to a susceptible host. In epidemiology, infectiousness describes the likelihood that an infected person will transmit the pathogen per contact with another person, and it can vary over the course of an illness.

Infectiousness is distinct from virulence or disease severity. A pathogen can be highly infectious but cause mild disease, or be less infectious yet highly virulent. It depends on pathogen traits (infectious dose, stability, replication rate), host factors (immune status, behavior), and environmental settings (crowding, ventilation, hygiene). Transmission routes—such as respiratory, fecal-oral, bloodborne, sexual, or vector-borne—also shape how infectiousness translates into spread.

Measures used in public health include the basic reproduction number (R0), which estimates the average number of secondary cases from a typical infected person in a fully susceptible population, and the effective reproduction number (Re) or Rt for current conditions. Other concepts include the secondary attack rate, generation time, and serial interval. The infectious period and the timing of peak infectiousness influence when transmission is most likely. Transmission is often heterogeneous; some individuals or events (superspreading) contribute disproportionately to spread.

Public health strategies aim to reduce infectiousness or contact rates, through testing, isolation, contact tracing, vaccination, masking, ventilation, and good hygiene. Patterns of infectiousness vary across diseases and contexts, illustrating the dynamic nature of pathogen transmission.