contagious

Contagious is an adjective used to describe a disease or condition that can be transmitted from one person to another, typically through direct contact, close proximity, or shared environments. The term derives from Latin contagiosus, from contāre “to touch,” reflecting historical emphasis on touch as a mode of transmission. In public health, contagious often refers to infections that spread between people, though not all infectious diseases are contagious.

Transmission routes include direct contact (skin-to-skin or mucous membranes), indirect contact via contaminated objects (fomites), respiratory droplets produced by coughing or sneezing, and airborne spread through smaller particles that can remain suspended. Some infections are also spread via vectors (such as insects) or through contaminated food or water, but such routes are not always described as contagious in common usage.

Not every disease is contagious. Non-contagious conditions do not readily pass from person to person, and some infections are non-transmissible outside specific circumstances. Contagiousness also varies with factors such as the pathogen's characteristics, the environment, and host immunity. The basic reproduction number, R0, is used to estimate how many people a typical case could infect in a fully susceptible population; higher R0 values indicate greater contagiousness and usually greater public health challenges.

Public health measures to curb contagious spread include isolation of cases, contact tracing, hand hygiene, mask use in appropriate settings, vaccination, ventilation improvements, and safe food and water practices. Examples commonly described as contagious include measles, chickenpox, influenza, and SARS-CoV-2, though their levels of contagiousness differ substantially.