Almost 90% of diarrhea cases and 15% of under-5 diarrhea deaths worldwide could be prevented through improved water, sanitation, and hygiene. Sanitation interventions are also important for the prevention of soil-transmitted helminths, which infect 21 million children under five each year. Infection with soil-transmitted helminths and repeated episodes of diarrhea early in life can compromise physical and cognitive growth and development, resulting in poorer school performance later in life. Thus, water, sanitation, and hygiene (WASH) interventions are important not only for reducing child mortality, but also for preventing cycles of poverty and poor health.

It is possible that WASH interventions affect not only those who receive them but also those who are geographically proximate or connected socially to those receiving the intervention. Indeed, there is a large infectious disease modeling literature based on this premise. Investigators define intervention effects on non-recipients "spillovers", and they are often referred to as "herd effects" or "indirect effects". Most studies that have empirically measured spillovers of child health interventions with an experimental design have focused on vaccines and deworming, and no studies have measured spillovers from WASH interventions. The development and application of methodology for measuring spillovers of community interventions empirically would make a valuable contribution to fields including epidemiology, economics, political science, and social welfare, all of which are concerned with measuring the impact of programs and interventions which may spill over. The presence and magnitude of positive spillovers are important; if spillovers are present and are in the same direction as treatment effects but are not accounted for when estimating treatment effects, estimates will be biased towards the null. As a result, both the efficacy and cost effectiveness of the intervention will be underestimated.

In this study, investigators will measure spillovers of water, sanitation, and hygiene interventions in an existing, large, rigorously designed trial: the WASH Benefits trial (https://clinicaltrials.gov/ct2/show/NCT01590095). Funded by the Bill \& Melinda Gates Foundation, this trial aims to measure the individual and combined effects of water, sanitation, and hygiene interventions on child health and development. It is a cluster-randomized, controlled trial with six treatment arms and a double-sized control arm carried out in rural Bangladesh. This add-on study is funded by the National Institute for Child Health and Human Development (1R21HD076216-01A1). Investigators hypothesize that children who live in close proximity to compounds that receive a combined sanitation, handwashing, and water treatment intervention--compared to children who live in close proximity to control compounds (no intervention)--will have: 1) lower prevalence of diarrhea, 2) lower prevalence and intensity of infection of soil transmitted helminths, and 3) lower prevalence of respiratory illness.

Investigators will collect additional data from the existing combined intervention (sanitation+handwashing+water) and control arms of the WASH Benefits trial. For each WASH Benefits household, investigators will locate the nearest household with children 0-59 months of age that are not enrolled in WASH Benefits and collect data in that household. Our primary outcomes are soil transmitted helminth infection among children 0-59 months, caregiver-reported 7-day diarrhea, and respiratory illness among children 0-59 months (the same age as the WASH Benefits cohort). Our findings will document either the presence or absence of spillovers of the combined sanitation+handwashing+water intervention.