Contagious describes diseases or infections that can spread from one person to another through various means—direct contact, airborne droplets, contaminated surfaces, or other transmission routes. How contagious a disease is (its transmissibility) varies widely: measles is extremely contagious, spreading easily through air, while other diseases require close contact. Understanding contagiousness informs public health responses—highly contagious diseases may require isolation, quarantine, mask requirements, or vaccination campaigns. The COVID-19 pandemic heightened public awareness of disease transmission, leading to widespread adoption of measures to reduce contagion. Public health authorities track contagious disease spread and issue guidance to protect communities.