1. Any of several tick-borne infections of domestic and wild animals that are caused by protozoans of the genus Babesia.

2. A human disease caused by infection of red blood cells by protozoans of the genus Babesia that are transmitted by deer ticks, characterized by fever, malaise, and hemolytic anemia. In the United States, it occurs chiefly in the Northeast and Midwest. In both senses also called piroplasmosis.