The largest cyclospora outbreak in U.S. history has sickened up to 22,000 people and killed at least two, but investigators still can't pinpoint exactly how human fecal matter contaminated Mexican lettuce farms—revealing a food safety system chronically underfunded and reliant on companies to police themselves. Cyclospora is a parasite spread exclusively through food or water contaminated with human feces; standard chlorine treatment cannot kill it, making prevention exceptionally difficult.