A method of reducing symptoms associated with cryptosporidia infection in mammals including humans in which L. reuteri cells are administered to the mammal in an amount sufficient to reduce diarrhea in the animal to a normal level. Daily administration as soon as cryptosporidia infection symptom are observed optimizes results. The L. reuteri cells may also be administered prophylactically to a mammal before exposure to cryptosporidia. The cells may be administered suspended in a liquid or in dry form.