“But why would sick people smell differently in the first place? The key is that our bodies are constantly launching volatile substances into the air. They’re carried in our breath and literally ooze from every pore, and they can vary depending on age, diet, and whether an illness has thrown off some cog in our metabolic machinery. Microbes living in our guts and on our skin also contribute to our signature scent, by breaking down our metabolic by-products into smellier ones.
Basically, you’re a walking factory of smells. And if you start paying attention to them, you might notice when something’s off.”
This is an exert from the article on the National Geographic site
The article tells of a woman who can smell Parkinson’s which brought attention to the idea of sniffing for disease.
Anyway it is an article worth reading.