Two highly infectious bordetellae, Bordetella pertussis and B. parapertussis, have emerged in historical times as co-dominant in human populations. Both of these cause acute disease (whooping cough), whereas their progenitor, B. bronchiseptica, is of variable virulence in a wide variety of animals. The remarkably close phylogenetic relatedness of these three bordetellae and the two independent jumps to humans provide a unique opportunity to examine the evolution and genetics involved in the emergence of acute human pathogens. We hypothesize that the more virulent strains in humans reflects how acutely infectious pathogens might be favored in communities with large contact networks. Furthermore, we suggest that the differential expression of the various virulence factors by the two human pathogens can be explained by immune-mediated competition between the strains. The evolutionarily favored strategies of both of the human bordetellae result in immunizing infections and acute epidemics.