Rationale: This is an explosive outbreak of gastroenteritis in adult individuals who were present at a football game. A common-source transmission of a highly communicable agent should be suspected. Viral etiologies are among the most common causes of these outbreaks. Unfortunately, these often cannot be differentiated from other pathogens based on clinical grounds alone, and even routine testing is inadequate. Although bacterial pathogens could be responsible, protozoal causes in this situation would be distinctly unusual.