* Based on tracking of waterborne outbreaks from 1971-2010. Only confirmed causes have been included in the analyses. For outbreaks with multiple causes, each agent counted toward the total. Outbreak reporting is dependent on capacity to detect, investigate, and report the outbreaks. This requires health effects to be measured and these health effects to be easily linked to water exposure. Clusters of illnesses associated with chronic chemical exposures are not part of waterborne disease outbreak reporting or part of these lists.