Approximately 48 million people in the US are served by private, and frequently untreated, wells. Our best estimate is that 1.3 million cases of gastrointestinal illnesses (GI) per year are attributed to consuming water from untreated private wells in the US, but in reality, there are no robust epidemiological data that can be used to estimate cases of GI attributable to these sources. It is likely that well water-associated GI causes significant healthcare costs and lost work/school days, as well as increased risk for long term health complications. This impact is magnified when accounting for vulnerable populations such as children under the age of 5, the elderly, and the immunocompromised. We propose the first randomized controlled trial (RCT) to estimate the burden of GI associated with private well water. We will test if household treatment of private well water by ultraviolet light (UV) vs. sham (placebo inactive UV device) decreases the incidence of GI in children under 5. At present, there are no prior RCTs or studies that have sought causal links between GI and the consumption of untreated water from private wells despite the fact that pathogens have been recovered in groundwater, including deep aquifers. Under the guidance of an interdisciplinary advisory committee we will execute the following aims: Aim 1- Quantify the incidence rate of endemic childhood GI associated with consuming untreated private well water and compare that to the incidence rate of consuming well water treated by UV. Aim 1a- Construct a Quantitative Microbial Risk Assessment (QMRA) using water quality data we collect to estimate the risk of childhood GI associated with consuming untreated private well water and compare the incidence from the risk model to the incidence we calculate in Aim 1. Aim 2- Identify, quantify and compare viral, bacterial and protozoan pathogens in stool of children consuming UV treated or untreated (sham) private well water (including both asymptomatic and symptomatic cases). Aim 3- Explore the presence of pathogens in untreated well water and stool samples of children consuming untreated private well water (sham group only). These data will fill a knowledge gap on sporadic GI associated with federally-unregulated private water supplies in the US. Our results will test an affordable water treatment intervention and inform GI burden estimates and policy decisions for managing well water in the US and globally. Policy changes will help better protect rural families, especially children who are at highest risk for sporadic enteric infections.