a genus of protozoa of the order Piroplasmida, occurring as single or paired parasites within the erythrocytes of various vertebrates, causing babesiosis and other diseases in domestic and wild animals and humans and transmitted by ticks, in which a sexual multiplicative cycle occurs.

Certain species have been reported to cause a malarialike disease in both healthy and splenectomized individuals. Formerly called Babesiella and Piroplasma.