Poop is naturally smelly because of bacteria and compounds. But if it smells worse than usual and comes with other symptoms like diarrhea, belly cramps, or nausea, it could be a sign of an infection. Certain bacteria, viruses, and parasites can lead to stomach bugs. Giardiasis is a type of diarrhea that triggers unusually bad-smelling poop. The giardia parasite, typically found in untreated water and food, causes it.