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.
