The paper describes a modeling framework designed to capture the interdependent transmission pathways of enteric pathogens. Results suggest that the benefits of a water quality intervention depend on sanitation and hygiene conditions. When sanitation conditions are poor, water quality improvements may have minimal impact regardless of amount of water contamination. If each transmission pathway alone is sufficient to maintain diarrheal disease, single-pathway interventions will have minimal benefit. Ultimately an intervention will be successful only if all sufficient pathways are eliminated. American Journal of Public Health 2007;97:846-52.