In epidemiology, a virgin soil epidemic is an epidemic in which populations that previously were in isolation from a pathogen are immunologically unprepared upon contact with the novel pathogen. Virgin soil epidemics have occurred with European colonization, particularly when European explorers and colonists brought diseases to lands they conquered in the Americas, Australia and Pacific Islands.
When a population has been isolated from a particular pathogen without any contact, individuals in that population have not built up any immunity to that organism and also have not received immunity passed from mother to child. The 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, due to the potential benefits to immune system function due to genetic diversity. That can happen also when such a considerable amount of time has passed between disease outbreaks that no one in a particular community has ever experienced the disease to gain immunity. Consequently, when a previously unknown disease is introduced to such a population, there is an increase in the morbidity and mortality rates. Historically, that increase has been often devastating and always noticeable.
Diseases introduced to the Americas by Europeans and Africans include smallpox, yellow fever, measles and malaria as well as new strains of typhus and influenza.
Virgin soil epidemics also occurred in other regions. For example, the Roman Empire spread smallpox to new populations in Europe and the Middle East in the 2nd century AD, and the Mongol Empire brought the bubonic plague to Europe and the Middle East in the 14th century.
History of the term
The term was coined by Alfred Crosby as an epidemic "in which the populations at risk have had no previous contact with the diseases that strike them and are therefore immunologically almost defenseless." His concept is related to that developed by William McNeill, who connected the development of agriculture and more sedentary life with the emergence of new diseases as microbes moved from domestic animals to humans.
The concept would later be adopted wholesale by Jared Diamond as a central theme in his popular book Guns, Germs and Steel as an explanation for successful European expansion.
Historical instances
Native American epidemics
Due to limited interaction between communities and more limited instances of zoonosis, the spread of infectious diseases was generally hampered in Native American communities. This contrasted with Eurasia, where a large number domesticated animals in close contact with large human populations would lead to more frequent zoonotic diseases, which would then in turn spread between human populations more easily due to trade and warfare. Native Americans were not exposed to this latent pool of circulating Eurasian diseases until the European colonization of the Americas, which then led to frequent virgin soil epidemics among Native Americans.
Cocoliztli epidemics
A series of epidemics of unknown origin caused major population collapses in Central America in the 16th century, possibly due to little immunological protection from previous exposures. While the pathogenic agents of these so-called Cocoliztli epidemics are unidentified, suspected pathogenic agents include endemic viral agents, Salmonella, or smallpox.
Australian Aboriginal epidemics
The European colonization of Australia led to major epidemics among Australian Aboriginies, primarily due to smallpox, influenzas, tuberculosis, measles, and potentially chickenpox.
Other instances
With malaria spreading in the Caribbean islands after European-African contact, the immunological resistance of African slaves to malaria in contrast to the immunologically defenseless locals might have contributed to African slave trade.
Novel and rapid-spreading pandemics such as the Spanish flu are occasionally referred to as virgin soil pandemics.
Debate
Research over the last few decades has questioned some aspects of the notion of virgin soil epidemics. David S. Jones has argued that the term "virgin soil" is often used to describe a genetic predisposition to disease infection and that it obscures the more complex social, environmental, and biological factors that can enhance or reduce a population's susceptibility.
Paul Kelton has argued that the slave trade in indigenous people by Europeans exacerbated the spread and virulence of smallpox and that a virgin soil model alone cannot account for the widespread disaster of the epidemic.