J. O'Donnell. Addition: Actually, I've had a look at a more recent paper from the PNAS (Proceedings of the National Academy of Sciences) that suggests that bacteria delivered through a flea bite may directly cause scepticemia. I had assumed that the condition was simply a subclinical form of bubonic plague, where the bacteria still pass through the lymph nodes but do not cause direct infection. It appears that Y. pestis injected into a host through natural means, in this case a flea bite, may indeed cause a patient to proceed directly to a scepticemic form of plague.  The paper is free if anyone is curious to read it: http://www.pnas.org/cgi/reprint/103/14/5526 -J.