This micrograph shows the digestive tract dissected from a flea that had dined about two weeks previously on the blood of an animal infected with the plague bacterium, Yersinia pestis. The bacteria multiplied in the flea gut to produce large cohesive aggregates, indicated by the red arrows; the bacterial mass on the left is occluding the passage between the esophagus and the midgut. This type of blockage prevents a flea from digesting its blood meals, thereby causing it to bite repeatedly, disseminating the infection. (From B.J. Hinnebusch, E.R. Fischer and T.G. Schwann, J. Infect. Dis. 178:1406–1415, 1998. © The University of Chicago Press.)