Babesiosis is caused by a tick-borne hemoparasite that, like malaria, can cause fever, hemolysis, and anemia. Typically self-limited, in the asplenic, immunocompromised, or elderly, disease can be severe or deadly. US cases have been primarily due to Babesia microti; WA-1, which may be related to Babesia gibsoni; and MO-1, related to Babesia divergens. European infections are usually due to B. divergens. North American cases are treated either with quinine and clindamycin or with atovaquone and azithromycin. The latter regimen appears less toxic.