Dogs are truly amazing. From Study Shows Dogs Able to Smell Cancer:
It is thought that a dog’s sense of smell is generally 10,000 to 100,000 times better than a human’s.The idea that dogs may be able to smell cancer was first put forward in 1989 by two London dermatologists, who described the case of a woman asking for a mole to be cut out of her leg because her dog would constantly sniff at it, even through her trousers, but ignore all her other moles.
One day, the dog, a female border collie-Doberman mix, had tried to bite the mole off when the woman was wearing shorts.
It turned out she had malignant melanoma — a deadly form of skin cancer. It was caught early enough to save her life.
Another anecdote:
Then in 2001, two English doctors reported a similar case of a man with a patch of eczema on his leg for 18 years. One day his pet Labrador started to persistently sniff the patch, even through his trousers. It turned out he had developed skin cancer and, once the tumor was removed, the dog showed no further interest in the eczema patch.