The organism is an obligate intracellular pathogen, which must infect eukaryotic cells to multiply. Orientia tsutsugamushi is the causative organism of scrub typhus, and the natural vector and reservoir is probably trombiculid mites (genus Leptotrombidium). The envelope is similar to that of Gram-negative bacteria, but it is not easily stained with Gram stain and the Gimenez stain is preferred. A large number of serotypes are described: these include Karp (which accounts for about 50% of all infections), Gilliam (25%), Kato (less than 10%), and Kawasaki, but enormous variability exists, with eight serotypes being reported in a single field in Malaysia, and many more serotypes continue to be reported.