C. parvum Iowa II (IIa) strain oocysts were obtained from Bunch Grass Farms. A C. tyzzeri strain isolated from laboratory mice at Washington University in St. Louis was kindly provided by Dr. Chyi-Song Hsieh, University of Washington [39 (link)]. Parasites were maintained in 6–8 week old male and female Ifnγ−/− (Jackson Laboratory stock no. 002287) or C57BL/6 (Ct) mice bred in-house. 6–8 week old mice were handled, treated with antibiotics, and infected as detailed in [23 (link)].