A contentious hypothesis about where and how Parkinson’s disease starts off is gaining ground as new studies provide clues in its support. Neuroanatomists Heiko Braak and Kelly Del Tredici, both at the University of Ulm in Germany, have described the distribution of Lewy bodies—the hallmarks of PD pathology—in the central and peripheral nervous systems of deceased patients.

Based on their observations, they proposed in 2003 that the pathology begins in the intestinal nervous system. From there, they say, it progresses in predictable stages, first to the brainstem, then to the midbrain, and then upward to higher brain regions. Braak and del Tredici did not stop here. They went further to postulate that PD could be set off by a yet-unidentified pathogen, possibly a virus, entering the body from the nose and gut simultaneously. Writer Laura Bonetta describes the hypothesis, the main issues raised against it, and new research it has inspired.