Babesiosis is a zoonotic disease caused by an intraerythrocytic protozoan that is transmitted by Ixodes ticks. Babesiosis has been recognized as an emerging endemic disease in the northeastern and northern midwestern United States, with sporadic cases reported in the Far West and in Europe, Africa, and Asia. Although children are infected as frequently as are adults and may have serious episodes of disease, cases are reported more commonly in adults. The clinical spectrum of babesiosis ranges from asymptomatic infection to death. Typical clinical symptoms include fever, chills, sweats, headache, and fatigue. Patients at increased risk for severe disease include those who lack a spleen, are infected with human immunodeficiency virus, are older than 50 years, or are coinfected with the agents of Lyme disease or ehrlichiosis. Definitive laboratory diagnosis can be made by direct identification of the causative agent in Giemsa-stained peripheral blood smears and by serological evaluation. The combination of clindamycin and quinine is the current treatment of choice for babesiosis. Copyright © 2000 by W.B. Saunders Company