Gimenez stain of tick hemolymph cells infected with Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever. R. rickettsii is a small bacterium that grows inside the cells of its hosts. These bacteria range in size from 0.2 x 0.5 micrometers to 0.3 x 2.0 micrometers. They are difficult to see in tissues by using routine histologic stains and generally require the use of special staining methods.