In the case of the cyclospora parasite, federal health officials suspect that contaminated iceberg lettuce might be the source of the illnesses, but investigators are still exploring causes. When it comes to very unpleasant stomach bugs, norovirus causes terrible diarrhea , and it’s much more common and contagious than cyclospora, Bryan said. “Norovirus is responsible for the vast majority of diarrheal illnesses,” Bryan said. If infected animals defecate near crops, or farmers inadvertently irrigate produce with contaminated water, cyclospora parasites can attach to plants that humans later eat. You just need to wash produce well.