sMapBabesiosis is infectious disease of red blood cells caused by the one-celled parasite Babesia. Babesiosis is usually an asymptomatic infection in healthy individuals. In the United States, babesiosis usually affects people on the offshore islands or coastal regions of New York and Massachusetts. Babesiosis affects all age groups with similar frequency; however, patients older than 50 years are at increased risk for severe infection and death. Babesiosis is caused by hemoprotozoan parasites of the genus Babesia. This disease protozoan was first described in cattle in 1891 by the Hungarian microbiologist Babes. While more than 100 species have been reported, only a few have been identified as causing human infections. Babesiosis is limited to those who live in, or have recently traveled to, the northeastern United States. Babesiosis is most commonly spread to humans by the bite of a tick infected with the Babesia parasite. Babesiosis is spread by deer ticks, which are carried mainly by deer, meadow voles, and mice. Moderate to severe babesiosis may occur in children, but infection generally is less severe than in adults.