The negative control exposure analyses suggested that unmeasured sources of measurement or confounding bias were unlikely to explain the associations between Enterococcus and incident diarrhea. Excess incidence among swimmers occurred mainly within 2 days after exposure, but pathogen specific outcome measurements would be necessary to make definitive conclusions about etiology. Finally, we limited the analysis to gastroenteritis and related impacts, but extending similar analyses to additional endpoints such as ear infections or eye infections could increase estimates of public health burden. These limitations notwithstanding, the large study population, broad range of beach and water conditions, stability of the findings across many sensitivity and negative control analyses, and internal consistency of the results all lend credibility to our findings.