Ehrlichiosis is a disease caused by Ehrlichia organisms. It was first described in 1990. The two specific organisms that have been implicated are Ehrlichia chaffeensis and granulocytic Ehrlichia.

The Ehrlichia bacteria belong to the family Rickettsiae. Rickettsial organisms are responsible for a number of serious diseases with worldwide distribution such as Rocky Mountain spotted fever, typhus, murine typhus, scrub typhus, Queensland tick typhus, Boutonneuse fever, North Asian tick-borne rickettsiosis, rickettsial pox, and others. All of these diseases require an insect vector (tick, flea, mite) to transmit them to humans or other animals.

The infectious organism of ehrlichiosis is transmitted to humans by the bite of ticks. Rickettsiae are often associated with a specific tick. Ehrlichia inhabit the Lone Star tick (Amblyomma americanum), the American dog tick (Dermacentor variabilis), and the deer ticks (Ixodes dammini and Ixodes scapularis). (NOTE: the deer tick is also the carrier of Lyme disease.). The main geographic areas where ehrlichiosis is found in the United States are the southern central states and southern Atlantic states. It has recently been reported in the upper midwestern region.

Risk factor for ehrlichiosis include living in an area with a lot of ticks, activities in high grasses, and owning a pet that may bring tick home.