Rickettsiales

The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria that live as obligate intracellular parasites, unable to reproduce outside a host cell. The order includes Rickettsia, the cause of typhus and spotted fever diseases in humans, Ehrlichia, a pathogen of livestock, and Wolbachia, which infects roughly two thirds of all arthropod species and nearly all filarial nematodes. Their close relationship to mitochondria once supported the endosymbiotic theory of mitochondrial origin, though more recent genetic evidence suggests the two lineages diverged independently.