Ehrlichiosis and anaplasmosis are zoonoses caused by bacteria from the family Anaplasmataceae, including human and animal pathogens. The human pathogens are Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis (HME), Anaplasma phagocytophilum, the pathogen causing human granulocytic anaplasmosis (HGA), E. ewingii and Neorickettsia sennetsu, granulocytotropic and monocytotropic Ehrlichia species, respectively. Ehrlichia spp. are small, gram-negative, obligate intracellular bacteria. They replicate in the cytoplasmic vacuoles of host cells, especially granulocytes and monocytes, to form microcolonies called morulae. These agents are transmitted through the bite of infected tick. In the United States, the vectors are Amblyomma americanum, Ixodes scapularis and Ixodes pacificus ticks. The primary vector in Europe is Ixodes ricinus. Rodents, deer, roe deer, foxes, cattle, sheep, goats, horses and dogs are reservoirs of these bacteria in Europe. Peromyscus leucopus, the white-footed mouse, and Odocoileus virginianus, the white-tailed deer, are the most important reservoirs in the United States. Infection in humans is manifested as a nonspecific flu-like illness. The laboratory diagnosis is most frequently serological--evidence of antibody by indirect immunofluorescence assay (IFA) and detection of DNA by polymerase chain reaction (PCR), or microscopy evidence--Giemsa stain of blood smears (morulae in granulocytes or monocytes). Doxycycline is the drug of choice in therapy. Avoiding exposure to ticks is the best method of prevention of infection.