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 flu-like illness that usually lasts for a week or two but may require hospital admission. Those at greatest risk of this fatal disease include patients over 50 years of age, who lack a spleen or are immunocompromised 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, quinine, atovaquone, azithromycin and in severe cases, exchange transfusion.