恙虫东方体（Orientia tsutsugamushi）是恙虫病的病原体。其种小名「tsutsugamushi」源自于日语「つつがむし」，为日本汉字「恙虫」两字的发音。恙虫东方体的自然病媒及传染窝为恙螨（Leptotrombidium属）

恙虫东方体为胞内病原体，需寄生于真核细胞内才能繁殖。其外膜结构类似革兰氏阴性菌，但并不容易以革兰氏染色染色，必须采用Gimenez stain。恙虫东方体具有多种血清型，主要血清型包含 Karp（占总感染病例的半数），另外还有Gilliam（25%）、Kato（<10%），以及Kawasaki等。恙虫东方体的血清型变化相当多样 ，如马来西亚同时就有八种血清型通报。恙虫病棘手的部分在于感染某一血清型之后，不代表对于其他血清型会具有免疫力。另外恙虫东方体也可能在同一个体重复感染。

Orientia tsutsugamushi (from Japanese tsutsuga "illness" and mushi "insect") is the causative organism of scrub typhus, and the natural vector and reservoir is probably trombiculid mites (genus Leptotrombidium). The organism is an obligate intracellular pathogen, which needs to infect eukaryotic cells in order to multiply. 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. There are a large number of serotypes described: these include Karp (which accounts for approximately 50% of all infection), Gilliam (25%), Kato (less than 10%), and Kawasaki, but there exists enormous variability, with eight serotypes being reported in a single field in Malaysia, and many more serotypes continue to be reported. Genetic methods have revealed even greater complexity than had been previously described (for example, Gilliam is further divided into Gilliam and JG types). Infection with one serotype does not confer immunity to other serotypes (no cross immunity). Repeated infection in the same individual is therefore possible, and this complicates vaccine design.