contagion

Contagion is the transmission of an infectious agent from an infected person or reservoir to a susceptible host, with the potential for further spread within a population. It encompasses how easily a pathogen can move between hosts and depends on the biology of the agent as well as patterns of contact, behavior, and environment. Some pathogens are highly contagious, while others spread more slowly or require specific conditions or vectors.

Transmission occurs through multiple routes: direct contact (skin-to-skin, sexual, or close contact), indirect contact via contaminated objects or surfaces (fomites), droplet spread from coughing or sneezing within a short distance, airborne or aerosol transmission through small particles that can linger in air, vehicle-borne transmission via contaminated food or water, and vector-borne transmission by insects or other organisms.

Key epidemiological concepts include the incubation period (time from exposure to symptoms), the infectious period (when an infected person can transmit the agent), and the reproduction number R0, the average number of secondary cases generated by one case in a fully susceptible population. The effective reproduction number Rt reflects current transmission in a population and is influenced by immunity, behavior, and public health interventions.

Public health responses aim to interrupt transmission and reduce contagion. Measures include surveillance and case finding, isolation of cases, contact tracing, vaccination, hand hygiene, respiratory etiquette, ventilation improvements, and, when appropriate, social distancing or movement restrictions.

Contagion is also used to describe the rapid spread of ideas, behaviors, or emotions—social contagion—though the mechanisms differ from biological transmission.