Ehrlichia, a genus of small spherical to ellipsoidal, nonmotile gram-negative bacteria. They occur singly or in compact inclusions in circulating mammalian leukocytes. Some species are the causative agents of ehrlichiosis and are transmitted by ticks. Two human tick-borne diseases have been associated with Ehrlichia species: human monocytic ehrlichiosis caused by E. chaffeensis, and human granulocytic ehrlichiosis caused by E. equi.
