https://www.scientificamerican.com/article/a-new-way-to-detect-parkinsons-by-smell/

Scent has been used as a diagnostic tool by physicians for thousands of years. But smell tests are not common in modern medicine—when’s the last time you were smelled by your doctor or received a batch of smell results back from the lab? Now, new research suggests that odors can be used to screen for Parkinson’s disease, which currently is without a definitive diagnostic.

In the animal kingdom, scents emitted from a body often signal information about an individual’s mental or physical state. For example, stressed rodents have been shown to excrete distinctive odors. Human body odors also have this function, emitting a wide array of odor and non-odor related chemicals called volatile organic compounds. These compounds are emitted from different areas of the human body and vary with age, diet, sex, and possibly genetic background. Moreover, disease processes can influence our daily odor by changing these compounds.