A butcher develops a questionable habit of eating various kinds of raw, ground meat over several years. He eventually starts suffering from fatigue and lymphadenopathy. In his extensive physical examination, intensely white focal retinal lesions with vitritis are observed. Chorioretinitis is diagnosed, even though an older laboratory test, the Sabin-Feldman dye test, is found positive. This patient is found to be infected with which of the following?