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Having once gained access to the Grande Armée, Shigella bacteria were richly endowed to overcome the body’s defenses. A notable feature of dysentery is the small number of bacteria needed to infect a human host. In addition, Shigellosis provides patients who recover with no acquired immunity as a defense against further episodes. Nor is there crossover immunity from one species of the Shigella genus to another.
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Frank M. Snowden III (Epidemics and Society: From the Black Death to the Present)