Introduction: On Thursday, November 23, 2017, an outbreak of gastroenteritis was reported at a middle school in Loire-Atlantique, affecting more than 150 people. Various epidemiological, microbiological, and environmental investigations were conducted. Method: A retrospective cohort study of students and adults at the middle school was conducted using a questionnaire posted online the following day on the school’s website. Stool samples were tested successively for bacteria, viruses, and parasites. Environmental samples were collected for analysis. Results: The attack rate was 61% (180/293), with an age gradient ranging from 75% among 6th graders to 45% among adults. A peak of 75 cases was recorded on November 22. Consumption of water from the distribution network was not associated with the onset of the disease; being a day student was associated with it (relative risk, RR=1.6 [1.1-2.3]). Following negative bacteriological and virological analyses (15 samples), the use of a PCR detection method for 22 pathogens revealed the presence of the Cryptosporidium parvum IIaA15G2R1 parasite. Among the foods served six days prior (incubation period) to all half-boarders was organic cottage cheese made from unpasteurized milk. The highly transmissible C. parvum IIaA15G2R1 genotype, considered zoonotic, was found in fecal samples from calves near the cheese-making facility. Discussion: Human cryptosporidiosis appears to be significantly underdiagnosed in France. Routine protocols for the laboratory diagnosis of persistent diarrhea should be improved by including a parasitological screening (including for Cryptosporidium).