Transmission by fleabite is a recent evolutionary adaptation that distinguishes Yersinia pestis, the agent of plague, from Yersinia pseudotuberculosis and all other enteric bacteria. The very close genetic relationship between Y. pestis and Y. pseudotuberculosis indicates that just a few discrete genetic changes were sufficient to give rise to flea-borne transmission. Y. pestis exhibits a distinct infection phenotype in its flea vector, and a transmissible infection depends on genes that are specifically required in the flea, but not the mammal. Transmission factors identified to date suggest that the rapid evolutionary transition of Y. pestis to flea-borne transmission within the last 1,500 to 20,000 years involved at least three steps: acquisition of the two Y. pestis-specific plasmids by horizontal gene transfer; and recruitment of endogenous chromosomal genes for new functions. Perhaps reflective of the recent adaptation, transmission of Y. pestis by fleas is inefficient, and this likely imposed selective pressure favoring the evolution of increased virulence in this pathogen.