Human granulocytic anaplasmosis, a deer tick–transmitted rickettsial infection caused by Anaplasma phagocytophilum, is a common cause of undifferentiated fever in the northeast and upper Midwest United States. Patients are often initially diagnosed with a mild viral infection, and illness readily resolves in most cases. However, as many as 3% develop life-threatening complications and nearly 1% die from the infection. Although coinfections with Borrelia burgdorferi and Babesia microti occur, there is little evidence to suggest synergism of disease or a role for A phagocytophilum in chronic illness. No vaccine is available.