Fig. 2.

Flea-borne transmission of Y. pestis leads to two distinct forms of plague. (A and D) Lymph nodes from mice infected with wild-type Y. pestis showing the severe lymphadenopathy typical of bubonic plague, characterized by destruction of the normal tissue and its replacement with myriad Y. pestis. (B, C, E, and F) Normal-appearing lymph nodes from mice with primary septicemic plague caused by wild-type Y. pestis (B and E) or Pla⁻ Y. pestis (C and F). In primary septicemic plague, extracellular bacteria (green arrowheads) were intravascular (E) or localized to small peripheral clusters and associated with neutrophilic inflammation (F). (Scale bars: 0.5 mm; magnification: D and F, ×600; E, ×1,000.)