Illnesses often have their own signature odors, produced by their underlying pathologic processes. Diabetes in its late stages makes breath smell sweet, scurvy is marked by a putrid smell, and bacterial infections produce a variety of unique aromas. Given these odoriferous clues, it's not surprising that the nose has served as a key medical diagnostic tool for centuries.
In the 20th century, laboratory testing largely supplanted the physician's sense of smell, but recent years have seen renewed interest in this ancient technique—with a modern twist. One promising technology uses gas-sensing devices to detect volatile compounds from breath or urine. Proof-of-concept studies suggest that such devices may offer a relatively inexpensive, rapid, noninvasive method for detecting a variety of diseases, from pneumonia to lung cancer.
Friedrich MJ. Scientists Seek to Sniff Out Diseases. JAMA. 2009;301(6):585-586. doi:10.1001/jama.2009.90