There’s an excellent review paper published earlier this year in the International Journal of Epidemiology: Water, sanitation and hygiene for the prevention of diarrhoea by Sandy Cairncross, Caroline Hunt, Sophie Boisson, Kristof Bostoen, Val Curtis, Isaac Fung and Wolf-Peter Schmidt (all of the London School of Hygiene and Tropical Medicine, except the penultimate author who’s from the University of Georgia in Athens, GA). Here’s part of the Abstract:
Results: The striking effect of handwashing with soap is consistent across various study designs and pathogens, though it depends on access to water. The effect of water treatment appears similarly large, but is not found in few blinded studies, suggesting that it may be partly due to the placebo effect. There is very little rigorous evidence for the health benefit of sanitation; four intervention studies were eventually identified, though they were all quasi-randomized, had morbidity as the outcome, and were in Chinese.
Conclusion: We propose diarrhoea risk reductions of 48, 17 and 36%, associated respectively, with handwashing with soap, improved water quality and excreta disposal as the estimates of effect for the LiST model. Most of the evidence is of poor quality. More trials are needed, but the evidence is nonetheless strong enough to support the provision of water supply, sanitation and hygiene for all [emphasis added].