Ehrlichiosis is an infection of white blood cells that affects various mammals, including mice, cattle, dogs, and humans. It was first reported in dogs in 1935, and the first human case was documented in the United States in 1986. Ehrlichia are obligate, intracellular bacteria that are transmitted by ticks to humans. They grow as a cluster (morula) in neutrophils (Anaplasma phagocytophilum and E. ewingii) and in monocytes (E. chaffeensis). The infection may cause prolonged fever and general aches, and is characterized by leukopenia, cytopenia, and elevated liver transaminases. In the first week of infection, ehrlichiae can be detected by finding intracellular aggregates on the blood/body fluid smears and various other laboratory findings. Immunofluorescent antibodies (IFA) titers and PCR are generally needed for confirmation and a definitive diagnosis. Early diagnosis is necessary as antibiotic treatment with doxycycline is very effective.