It is estimated that 10 percent of the population will have at least one episode of infectious diarrhea each year. Although many diarrheal diseases are self-limiting, some may be chronic and require ongoing treatment. Disease manifestations range from almost asymptomatic to severe and—in some cases—deadly. Viruses, parasites, or bacteria may be the infectious agents causing these episodes. Primary modes of transmission for most gastroenteritis agents are fecal–oral and foodborne; however, some agents may be spread from person to person. Because these agents are so different, viral agents, bacterial agents, and parasitic agents are examined in separate chapters.