How did separate underground water and sewage systems reduce the spread of disease?

Separate underground water and sewage systems were essential for public health because they prevented the contamination of drinking water by human waste, which is the primary transmission route for waterborne diseases like cholera.123

Historically, many urban areas relied on privy vaults and cesspools that frequently overflowed or leaked into the ground near homes, contaminating local well-water supplies. Because cholera and other pathogens spread through the fecal-oral route—often when water or food is contaminated by human excrement—this proximity between waste and drinking sources created a cycle of infection.123

By implementing dedicated sewers to transport waste away from residential areas and establishing separate, treated water supply systems, cities successfully broke the link between sewage and drinking water. This infrastructure, combined with later advancements like water purification through filtration and disinfection, led to the virtual elimination of deadly waterborne diseases in developed nations.14