Ehrlichiosis is the collective name for infections caused by obligate intracellular gram-negative bacteria in the genera Ehrlichia, Anaplasma, and Neoehrlichia, family Anaplasmataceae. Members of these genera cycle between invertebrate (arthropod) and vertebrate hosts, and some cause human zoonoses. At least seven species cause human tick-borne infection in the United States and Europe, including Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis (HME), Ehrlichia ewingii, the agent of human ewingii ehrlichiosis (HEE), Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis (HGA), an Ehrlichia muris-like agent (EMLA), the Panola Mountain ehrlichia, an agent phylogenetically similar to Ehrlichia ruminantium that has caused fever in humans in the United States, Ehrlichia canis, thought limited to canids but identified as an agent of human febrile illness in Venezuela, and Candidatus Neoehrlichia mikurensis, which has caused severe sepsis-like conditions in Europe, but mild febrile disease in Asia. While human infection by Neorickettsia sennetsu periodically surfaces in Asia, the transmission and disease processes are distinct and it will not be considered here.

Most Anaplasmataceae reside in ixodid (hard-body) ticks, and the bacteria are acquired during the larval stage and passed transstadially with each successive tick stage. Amblyomma americanum (the Lone Star tick) is the vector for E. chaffeensis and E. ewingii, and its range is throughout the south and eastern United States from Maine to Texas. In addition, all documented reports of human infections are limited to North America, although some evidence suggests that they exist in ticks in South America and Asia. In contrast, A. phagocytophilum and the E. muris-like agent cycle within Ixodes species ticks. Ixodes scapularis (the black-legged or deer tick) is found in the eastern United States and is a vector for both species. In addition, Ixodes pacificus (the western black-legged tick), found in regions of the US Pacific coast (northern California, Oregon, and Washington), and Ixodes ricinus and Ixodes persulcatus, found in Europe and Asia, respectively, are competent vectors for A. phagocytophilum. Ixodes species ticks are also vectors for Borrelia burgdorferi (the agent of Lyme borreliosis), and most cases of HGA are reported from areas where Lyme borreliosis is endemic.