Y. pestis has been subdivided into three phenotypic biovarieties—Antiqua, Medievalis, and Orientalis. Based on epidemiological and historical records, it has been hypothesized that Antiqua, presently resident in Africa, is descended from bacteria that caused the first pandemic, whereas Medievalis, resident in central Asia, is descended from bacteria that caused the second pandemic; those of the third pandemic, and currently widespread, are all Orientalis. It is believed that Y. pestis probably evolved during the last 1500 to 2000 years because of changes in social and economic factors that were themselves the result of a dramatic increase in the size of the human population, which was coincident with the development of agriculture.
This increase in food supply for humans allowed rodent populations to expand as well. Increased numbers of rodents coupled with changes in behavior (i.e., living in and around sylvatic rodents and human habitation) triggered the evolution of virulent Y. pestis from the enteric, food-borne, avirulent pathogen Y. pseudotuberculosis. This occurred by means of several genetic changes. For example, development of a gene whose product is involved in the storage of hemin resulted in the ability of the bacteria to block the flea proventriculus, enhancing flea-mediated transmission. Other gene products (phospholipase D and plasminogen activator) facilitated blood dissemination in the mammalian body and allowed for the infection of a variety of hosts by fleas.
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