A couple of drops in a swimming pool. That's one description of how good a dog's sense of smell is: a dog can detect the presence of something even when that something is surrounded by other substances.
That's why we use dogs to sniff out drugs and bombs... and diseases. It's true, dogs are getting more work in medical laboratories, being trained to detect the presence of things like cancer cells.
Maria Goodavage tells the story in the book Doctor Dogs: How Our Best Friends Are Becoming Our Best Medicine.
Grab the family dog, if you have one, and listen to our interview from 2020.