Animal models for studying immune responses to Cryptosporidium, a parasite that causes gastrointestinal disease, have been a challenge due to the parasite's poor infectivity in mice. Russler-Germain et al. discovered a 'commensal' strain of Cryptosporidium, capable of stable infection and vertical transmission, that elicits a T helper type 1 (Th1) response to promote intestinal homeostasis.