Anaplasma phagocytophilum is an obligate intracellular bacterium most commonly acquired from tick bites. It the causative agent of human granulocytic anaplasmosis transmitted by Ixodes scapularis in New England, the North Central Atlantic states, Wisconsin, and Minnesota. Ixodes pacificus is the vector on the West coast. These are the same ticks that causes Lyme disease and babesiosis and the endemic areas overlap. Ticks can be infected with more than one disease and transmit them both. Most cases are reported in the months from April to September.