Babesiosis is caused by microscopic parasites that infect red blood cells. Most human cases of Babesia infection in the United States are caused by the parasite Babesia microti. Occasional cases caused by other species (types) of Babesia have been detected. Babesia microti is spread in nature by Ixodes scapularis ticks (also called blacklegged ticks or deer ticks). Tickborne transmission is most common in particular regions and seasons: it mainly occurs in parts of the Northeast and upper Midwest; and it usually peaks during the warm months. Babesia infection can range in severity from asymptomatic to life threatening. The infection is both treatable and preventable.