Ehrlichiosis is the collective name for infections caused by obligate intracellular gram-negative bacteria in the genera Ehrlichia and Anaplasma that belong to the family Anaplasmataceae. Members of these two genera cycle in nature between invertebrate (arthropod) and vertebrate (mammalian) hosts, and some species occasionally cause zoonotic infections in humans. Three species are currently known to cause human tick-borne infection in the United States and Europe and include Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis (HME), Ehrlichia ewingii, the agent of human ewingii ehrlichiosis (HEE), and Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis (HGA). In nature Ehrlichia and Anaplasma species reside in specific hard-body tick hosts, and the bacteria are passaged transstadially with each successive developmental tick stage. Amblyomma americanum (the Lone Star tick) is the tick vector for E. chaffeensis and E. ewingii, and the endemic range of the Lone Star tick is predominately in the south and southeastern United States from Maryland to Texas. In addition, all documented reports of human infections with these species have been limited to the North American continent. In contrast, A. phagocytophilum cycles within Ixodes species ticks, including Ixodes scapularis (the deer tick) in the eastern United States, Ixodes pacificus (the black-legged tick) in some regions of the U.S. Pacific coast (northern California, Oregon, and Washington), and Ixodes ricinus (the wood tick) and Ixodes persulcatus in Europe and Asia. Ixodes species ticks are also vectors for Borrelia burgdorferi (the agent of Lyme borreliosis), and most cases of HGA have been reported from the same areas where Lyme borreliosis occurs endemically.