Rickettsia prowazekii is the pathogen of epidemic typhus. Its epidemic potential caused it to be listed as a human threat in the Select Agent Program, and in CDC Bioterrorism Agents-Disease list Category B.The organism appears to have a fundamental niche in the development of the cells of higher organisms. The functional profiles of its genes show similarities to those of mitochondrial genes: no genes required for anaerobic glycolysis are found in either R. prowazekii or mitochondrial genomes, but a complete set of genes encoding components of the tricarboxylic acid cycle and the respiratory-chain complex is found in R. prowazekii.
In effect, ATP production in Rickettsia is the same as that in mitochondria....Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than is any other microbe studied so far. 
The organism's reservoir is the alimentary tract of lice. When a louse carrying R. prowazekii bites a human to drain blood for its own nutrition, the louse defecates as it eats. Since the bite causes itching, the human victim scratches the site, crushes the louse, and drives the excrement into the skin. There is no direct transmission into human blood by an untouched louse.
If the louse does survive the human encounter, it is still doomed, although it acquired R. prowazekii from its bite. Of the arthropod vectors of the various forms of typhus, the louse is the only one that is doomed by the pathogen.  The relationship between the louse and R. prowazekii is, in their small world, rather reminiscent of Greek tragedy; the outcome differing only to the extent by which the louse enjoyed, in its lousy way, its meals.
Like all Rickettsiae, it is a small pleomorphic organism that can assume various shapes, such as a coccus or bacillus. It is cultured and stained only with difficulty, so the diagnosis is principally clinical, and confirmed by various immunologic mechanisms.
- Andersson SGE et al. (12 November 1998), "The genome sequence of Rickettsia prowazekii and the origin of mitochondria", Nature 396: 133-140, DOI:10.1038/24094
- Okulicz JF et al. (May 12, 2006), "Typhus", eMedicine