Yersinia pestis

The organism is a facultative intracellular pathogen that uses a type III secretion system to inject Yop effector proteins, disabling phagocytosis and inflammatory signalling; the F1 capsular antigen is also antiphagocytic. Xenopsylla cheopis, the rat flea, becomes blocked by a bacterial biofilm in its foregut, regurgitating organisms while feeding. In the United States plague is enzootic among prairie dogs, ground squirrels, and their fleas in the south-western states. Bubonic plague follows a flea bite with an abrupt high fever and an exquisitely tender bubo in the draining node; septicaemic plague brings disseminated intravascular coagulation with acral necrosis and gangrene, historically the 'black death'; pneumonic plague, whether primary from inhalation or secondary from haematogenous spread, is rapidly fatal and is the only person-to-person transmissible form, requiring droplet isolation and prophylaxis of contacts. It is a category A bioterrorism agent. Treatment is streptomycin or gentamicin, with doxycycline or ciprofloxacin as alternatives.