Virgin soil epidemic refers to an epidemic resulting from the introduction of a disease into a place where it does not occur or spread naturally. Virgin soil epidemics have occurred with European colonization, particularly during the Age of Discovery, when European explorers and colonialists brought diseases to "New World" lands in the Americas, Australia and Pacific Islands.[1]
When a population has not had contact with a particular pathogen, individuals in that population have not built up any immunity to that organism, and have not received immunity passed from mother to child.[2] Epidemiologist Francis Black has suggested that some isolated populations may not have mixed enough to become as genetically heterogeneous as their colonizers, which would also have affected their natural immunity.[2] Consequently when a previously unknown disease is introduced to such a population, there is an increase in the morbidity and mortality rates; historically this increase has often been devastating, and always noticeable.[1]
Diseases introduced to the Americas by European colonizers—and their African slaves—include smallpox, yellow fever, measles and malaria, as well as new strains of typhus and influenza.[3][4]
Virgin soil epidemics also occurred in the Old World prior to the Age of Discovery. For example, the Romans spread smallpox through new populations in Europe and the Middle East in the 2nd century AD, and the Mongols brought the bubonic plague to Europe and the Middle East in the 14th century.[4]