Herd Effect

The term originated in veterinary science before migrating to human medicine — the Bureau of Animal Industry used it as early as 1916 for disease resistance in cattle. British bacteriologists W.W.C. Topley and G.S. Wilson gave it its first rigorous human-disease definition in their 1923 paper in the Journal of Hygiene, using experimental mouse populations to argue that a population's disease resistance was an irreducibly collective phenomenon. Mass vaccination campaigns in the mid-20th century turned the concept into a core public-health tool.