This 1976 transmission electron microscopic (TEM) image depicted a hypertrophic peritoneal mesothelial cell from a mouse that had been experimentally infected with Orientia tsutsugamushi rickettsial micro-organisms. In this TEM, several organisms were visible, free within the host cell's cytoplasm. One O. tsutsugamushi appeared within a phagocytic vacuole, still bearing a third outer membrane layer of probable host cell origin. Formerly known as Rickettsia tsutsugamushi, O. tsutsugamushi is the pathogen responsible for causing the febrile disease known as scrub typhus. The disease is transmitted to humans through the bite of larval trombiculid mites, i.e., chiggers, that had fed on infected rodents.