As scientists studying DNA we learn a lot about mitochondria (circular DNA within eukaryotic cells responsible, in most part, for energy production). We are taught that the most likely origin of mitochondria was when one single-celled organism engulfed another creating an endosymbiont (cell living within a cell). There are multiple theories about the engulfing versus the engulfed cells but the typical explanation is an anaerobic (can live without oxygen) bacteria was engulfed by a nucleus-bearing cell. This cell-in-a-cell combo evolved to be able to create its own energy and later developed the capability to become multicellular organisms.
So, this ancestral species evolved into both mitochondria and R. prowazekii: mitochondria when it became fully incorporated into the other cell (mutual symbiosis) and R. prowazekii when it became a parasite (non-mutual symbiosis). In all of the times we have been taught about the origins of mitochondria, it was never mentioned that the common ancestor isn't just any bacteria but the common ancestor of a parasite. It puts a whole new dynamic on the origin story and the evolution of symbiotic relationships as well.
The original statement was found on Wikipedia but was verified by this scientific publication:
Emelyanov, Victor V. (2003) Mitochondrial connection to the origin of the eukaryotic cell. European Journal of Biochemistry 270(8): 1599-1618