Babesiosis is an emerging infection caused by protozoal parasites and transmitted by the same tick that transmits Lyme disease. Babesiosis is found throughout the world, but most cases have been described from the northeastern and northern midwestern United States. Patients experience a flulike illness that usually lasts for 1 or 2 weeks but may require hospital admission. Those at greatest risk of fatal disease include individuals older than age 50 years; asplenic individuals; and immunocompromised individuals as a result of immunosuppressive drugs, malignancy, or HIV infection. Specific diagnosis is made through examination of a Giemsa-stained thin blood smear, DNA amplification using polymerase chain reaction, or detection of specific antibody. Treatment consists of clindamycin and quinine or atovaquone and azithromycin and, in severe cases, exchange transfusion.