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Microbiology, 2021

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25.2 • Bacterial Infections of the Circulatory and Lymphatic Systems 1053<br />

Figure 25.9 Yersinia pestis, the causative agent of plague, has numerous modes of transmission. The modes are divided into two<br />

ecological classes: urban and sylvatic (i.e., forest or rural). The urban cycle primarily involves transmission from infected urban mammals<br />

(rats) to humans by flea vectors (brown arrows). The disease may travel between urban centers (purple arrow) if infected rats find their way<br />

onto ships or trains. The sylvatic cycle involves mammals more common in nonurban environments. Sylvatic birds and mammals (including<br />

humans) may become infected after eating infected mammals (pink arrows) or by flea vectors. Pneumonic transmission occurs between<br />

humans or between humans and infected animals through the inhalation of Y. pestis in aerosols. (credit “diagram”: modification of work by<br />

Stenseth NC, Atshabar BB, Begon M, Belmain SR, Bertherat E, Carniel E, Gage KL, Leirs H, and Rahalison L; credit “cat”: modification of<br />

work by “KaCey97078”/Flickr)<br />

Figure 25.10<br />

(a) Yersinia pestis infection can cause inflamed and swollen lymph nodes (buboes), like these in the groin of an infected

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