Daisy the Labrador had been training as a Medical Detection Dog with Dr. Claire Guest, when Daisy began to act in a strange and anxious manner. This training enables dogs to detect cancer by sniffing patients urine, breath and skin.

 
Guest noticed that Daisy kept jumping on her which was unlike her usual placid self, "One day she bumped into my chest with her nose. It was unusually sore, and there seemed to be a lump there. I had a fine needle biopsy, but it came back clear."

Daisys strange behaviour continued, promoting Guest to have further tests, which later revealed she had breast cancer. After undergoing treatment she has now been given the all clear.

Dr. Guest revealed that "Had I not had Daisys early warning, I would have had a very poor prognosis, she said. I have my dog to thank for the fact that Im standing here today."
(source)