Adaptation to host species probably involves sensing the unique features of their intracellular environment and responding by producing secreted effectors that have actions, which are specific to their milieu. Because so little research has been done outside of the L. pneumophila species, we are unable to annotate most of the molecular machinery used in these other diverse Legionella species. Legionella host adaptation may parallel the situation found in Chlamydiaceae, another group of pathogens with a broad and diverse range of hosts, where adaptation is driven primarily by amino acid change in conserved proteins and secreted effectors under strong diversifying selection58. Indeed, a large number of diverse effectors have recently been identified among 38 Legionella species19.