Rickettsiales
The Rickettsiales is a genetically diverse group of the alpha-Proteobacteria. They include major mammalian pathogens, such as the agents of epidemic typhus, scrub typhus, ehrlichioses and heartwater disease.
Sequenced genomes of this bacterial order have provided insights into reductive genome evolution, antigenic variation and host cell manipulation.
Recent results suggest that human pathogens emerged relatively late in the evolution of the Rickettsiales.
There is no association between pathogenicity and the acquisition of novel virulence genes.
References
Darby AC, Cho NH, Fuxelius HH, Westberg J, Andersson SG. Intracellular pathogens go extreme: genome evolution in the Rickettsiales. Trends Genet. 2007 Oct;23(10):511-20. PMID: #17822801#