This short National Geographic video here shows how dogs can smell certain diseases and can be trained to help those in need. One researcher in the video talks about how one day we humans might even be able to use the dogs' sniffing skills to diagnose disease.
Chapter Four of the Huang Di Nei Jing had stated thousands of years ago that a medical diagnosis could be made or confirmed by the presence of certain smells, ones that humans can detect. According to the five phases theory:
In the five odours, the odour of liver is stink. (urine)
In the five odours, the odour of heart is scorching.
In the five odours, the odour of spleen is fragrance.
In the five odours, the odour of lung is stinking. (fishy)
In the five odours, the odour of kidney is rancid. (rotten)
The above was quoted from: Yellow Emperor's Canon of Internal Medicine by Wang Bing (Tang Dynasty), translated by Wu Liansheng and Wu Qi.
I've found that after I leave a patient in the room with the door closed, their distinctive smell is always much more noticeable when I come back. I've definitely smelled the scent of burnt or scorched, but I haven't yet encountered a person smelling like stink and urine during treatment. That's probably a good thing.