Investigations into disease outbreaks generally incorporate an epidemiologic investigation, laboratory analysis, and an environmental health assessment. This last component is designed to discover connections between factors in the environment and the outbreak, but is often limited, either by time and resources, or the expertise of the personnel included in outbreak investigation teams. A waterborne Norovirus outbreak investigation in Sheridan County, Wyoming, in 2001 provides an excellent example of the importance of including an in-depth, systems-based environmental health assessment in outbreak investigations. The epidemiologic component of this investigation identified the water supply of a snowmobile lodge in the Bighorn Mountains as the source of the outbreak, a result that was confirmed by laboratory analysis. Including a systems-based environmental health assessment in this investigation also helped to uncover the underlying environmental factors that led to contamination of the water supply. Those factors included an onsite wastewater disposal system that was overloaded by increased use and not well suited to local soil and geologic conditions and a drinking water system with no treatment or disinfection. In addition, heavy precipitation and increased pumping of wells to satisfy higher demands probably facilitated the contamination of the drinking water wells by causing greater movement of wastewater through the soil and underlying bedrock. By focusing on these links between factors in the environment and adverse health outcomes, the systems-based environmental health assessment also helped to highlight prevention strategies for avoiding recurrences.