To review the role of the gastrointestinal tract in PD in a digestible manner, we will take a tour through the GI tract starting with discussion of the gut microbiota in the large intestine, followed by Helicobacter pylori in the stomach, small intestinal bacterial overgrowth in the small bowel, and ending with viruses and prion-like alpha synuclein in the gut-associated lymphoid tissue (Figure 1). This evidence concerns the gene SNCA and Parkinson disease.