Long before the biological mechanisms behind infectious disease transmission were scientifically understood, human societies had already developed practical strategies for limiting the spread of illness through the deliberate isolation of the sick or potentially exposed. One of the earliest formalised systems emerged in medieval Mediterranean port cities, where authorities began requiring ships arriving from regions experiencing plague outbreaks to wait offshore for a set period before passengers and cargo were permitted to disembark, a practice from which the modern term 'quarantine' derives its name. This waiting period was based on the practical, empirically derived observation that individuals who remained healthy throughout the isolation period were unlikely to be carrying the disease, even without any formal understanding of the microorganisms responsible for causing it. Quarantine practices gradually became more sophisticated and widespread over subsequent centuries, eventually incorporating designated isolation facilities, known as lazarettos, specifically built to house and treat individuals suspected of carrying contagious disease. The scientific validation of quarantine's underlying logic came only much later, following the 19th-century development of germ theory, which finally provided a coherent biological explanation for why isolating infected or potentially infected individuals could so effectively interrupt the ongoing chain of disease transmission. Quarantine measures, in various updated and adapted forms, remain an important tool of public health policy today, though modern understanding of specific disease characteristics, including incubation periods and precise transmission routes, now allows for far more precisely targeted and evidence-based interventions than the broader, more generalised measures of earlier centuries.