an acute infectious disease characterized by fever, chills, headache, joint and muscle pains, and sometimes respiratory, gastrointestinal, hepatic, or other systemic involvement; first described in 1994 in northeastern and northern midwestern states and in California. The causative agent, Anaplasma phagocytophilum, is an obligately intracellular organism with a gram-negative cell wall. The deer tick, Ixodes scapularis, is the principal vector, and the peak incidence is in July. Hematologic studies show depression of erythrocytes, leukocytes, and platelets. Clusters of developing organisms called morulae may be seen in neutrophils in stained blood smears, but serologic testing is more sensitive.human ehrlichiosis, human monocytotropic ehrlichiosis