One condition that can cause yellow poop is an infection known as giardia. Giardia are tiny organisms that are also parasites. Parasites are organisms that live in or on another organism to obtain nourishment. If giardia infects the intestines it can cause severe yellow diarrhea. This is a dangerous infection that can spread to others. In fact doctors that see it have to report it to the government's Center for Disease Control and Prevention.