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Theories of the Black Death
A number of epidemiologists and historians since the 1980s have challenged the traditional view that the Black Death was caused by plague based on the type and spread of the disease. The confirmation in 2010 and 2011 that ''Yersinia pestis'' DNA was associated with a large number of plague sites has renewed focus on plague as the leading hypothesis, but has not yet led to a final resolution of all these questions.
==Bubonic plague theory==
Several possible causes have been advanced for the Black Death; the most prevalent is the Bubonic plague theory. Efficient transmission of ''Yersinia pestis'' is generally thought to occur only through the bites of fleas whose mid guts become obstructed by replicating ''Y. pestis'' several days after feeding on an infected host. This blockage results in starvation and aggressive feeding behaviour by fleas that repeatedly attempt to clear their blockage by regurgitation, resulting in thousands of plague bacteria being flushed into the feeding site, infecting the host. However, modelling of epizootic plague observed in prairie dogs, suggests that occasional reservoirs of infection such as an infectious carcass, rather than "blocked fleas" are a better explanation for the observed epizootic behaviour of the disease in nature.
An interesting hypothesis about the epidemiology—the appearance, spread, and especially disappearance—of plague from Europe is that the flea-bearing rodent reservoir of disease was eventually succeeded by another species. The Black Rat (''Rattus rattus'') was originally introduced from Asia to Europe by trade, but was subsequently displaced and succeeded throughout Europe by the bigger Brown Rat (''Rattus norvegicus''). The brown rat was not as prone to transmit the germ-bearing fleas to humans in large die-offs due to a different rat ecology.〔Appleby, Andrew B. "The Disappearance of the Plague: A Continuing Puzzle", Economic History Review 33, 2 (1980) 161–173〕〔Slack, Paul. “The Disappearance of the Plague: An Alternative View.” Economic History Review 34, 3 (1981) 469–476.〕 The dynamic complexities of rat ecology, herd immunity in that reservoir, interaction with human ecology, secondary transmission routes between humans with or without fleas, human herd immunity, and changes in each might explain the eruption, dissemination, and re-eruptions of plague that continued for centuries until its unexplained disappearance.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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