BABESIOSIS - A rare, often severe (and sometimes fatal) illness in man that is caused by infection with the protozoal organism, Babesia microti. The disease, transmitted by ticks, occurs mostly in the Northeastern United States. The protozoan invades red blood cells causing fever, chills, sweats, joint pains, nausea, vomiting, and red blood cell hemolysis (rupture) leading to anemia.