Though the kinds of infections people came down with at treated and untreated locations varied, there was one commonality: poop. Human poop. Most of the pathogens causing problems get transmitted via the fecal-oral route, meaning an infected person swims in water and the bug travels from their anus into the water, and then someone else either swallows that water or inhales aerosolized droplets of it. For instance, about 80 percent of the illnesses contracted from treated water were caused by Cryptosporidium, a parasite that lives in animal intestines and spreads by shedding itself from feces into water sources. The illnesses from untreated water were more varied, but the main culprits still tended to come from poop. Norovirus was the single biggest contributor, at about 30 percent of all cases, and that's spread through vomit and feces. The most common bacterial infection, Shigella, spreads through diarrhea.