A method of reducing the severity of toxoplasmosis resulting from infection of a patient with Toxoplasma gondii by administering to a patient in need of such treatment, either after infection or before exposure to infection, a therapeutically effective amount of a compound that is a spiropiperidyl derivative of rifamycin S, wherein the derivative comprises an imidazole ring that includes carbons at positions 3 and 4 of the rifamycin ring, the carbon at position 2 of the imidazole ring also being a ring carbon at position 4 of a piperidine ring system, thereby forming a spiropiperidyl ring system, the spiropiperidyl ring system optionally comprising a lower hydrocarbon substituent on the nitrogen of the piperidine.