First reported by Victor Babes in 1888, Babesia microti is an intracellular parasite that infects red blood cells. The infection caused by this organism is known as babesiosis. Originally, Babes had thought the infection was due to the bacterium Haematococcus bovis, however, Theobald Smith and Fred Kilborne correctly identified the parasite as the causative agent in 1893. It was also first believed that this parasite only infected cattle, but again, they were proven wrong after observing a case within a splenectomised patient.
For infections occurring within the United States, the vector of the disease is the tick of the Ixodes species, also the vector of Borrelia burgdorferi , the bacterium that causes Lyme disease. For this reason, coinfections of babesiosis and Lyme disease are common.