Human babesiosis is an increasing health concern in the northeastern United States, where the causal agent, Babesia microti, is spread through the bite of infected Ixodes scapularis ticks. We sampled 10 mammal and 4 bird species within a vertebrate host community in southeastern New York to quantify reservoir competence (mean percentage of ticks infected by an individual host) using real-time PCR. We found reservoir competence levels>17 % in whitefooted mice (Peromyscus leucopus), raccoons (Procyon lotor), short-tailed shrews (Blarina brevicauda), and eastern chipmunks (Tamias striatus), and <6 % but>0 % in all other species, including all 4 bird species. Data on the relative contributions of multiple host species to tick infection with B. microti and level of genetic differentiation between B. microti strains transmitted by different hosts will help advance understanding of the spread of human babesiosis. Human babesiosis is a growing public health concern, especially in the northeastern United States. Babesiosis is a zoonotic, malaria-like illness that can be particularly severe and sometimes fatal in elderly, asplenic, or immunocomprised persons (1). In the lower Hudson Valley region of New York State, 5 locally acquired cases of babesiosis were documented in 2001 (2), and incidence has increased 20-fold from 2001 through 2008 (3). The causal agent of human babesiosis, Babesia microti (Apicomplexa: Piroplasmida), is a protozoan blood parasite that is transmitted in nature by the bite of an infected tick. In the northeastern United States, the vector of this disease is Ixodes scapularis, the black-legged tick, which is also the primary vector of Borrelia burgdorferi and Anaplasma phagocytophilum, the causal agents of Lyme disease and human granulocytic anaplasmosis, respectively. B. microt
