Babesiosis is an increasingly common, sometimes severe, and occasionally fatal disease caused by a parasite that infects red blood cells. It is caused by a single-celled parasite named Babesia. The disease can cause fever, fatigue, and anemia lasting from several days to several months. Symptoms tend to be more apparent and severe in elderly or immunocompromised persons. Infections can also occur without producing symptoms. In the United States, the disease occurs primarily in the upper Midwest and northeastern states. The vast majority of cases are acquired through the bite of an infected Ixodes scapularis tick (deer tick); however, in rare instances, the infection has been acquired through blood transfusions.