We report a fatal case of acute gastroenteritis in a child with autism spectrum disorder. Multiple viral coinfections were detected by PCR in the patient's stool and digestive biopsy specimens. As viral detection is not necessarily associated with symptomatic disease, a semi-quantitative approach using cycle treshold values was proposed for the clinical interpretation of PCR. We discuss whether concomitant viral infections could be a risk factor for severe outcome in gastroenteritis cases. Individual risk factors are also addressed.