The Rickettsiales, also called rickettsias, are an order of small proteobacteria. Most of those described survive only as endosymbionts of other cells. Some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans. On the other end of the scale, genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group. Some have also speculated that viruses might have developed from them, or from organisms like them.
The Rickettsiales are among the most mysterious groups of Proteobacteria, owing largely to difficulties in cultivating them. Originally the group included all obligate endosymbiont bacteria. However, a number of species have been removed, such as Coxiella burnetii, the cause of Q fever. Environmental samples have found a variety of genes that originate from this group, pointing to a number of free-living planktonic species. These include SAR11, later cultivated as Pelagibacter ubique, one of the most common bacteria in the world. As of 2005, the exact composition and classification is far from settled.