Those of us who own dogs know that they watch us all the time and read our body language like a sixth sense. In fact, they often read visual cues that we don't even realize that we are putting out into the world. More amazingly though they smell our bodies for changes.
Most oncologists will stand by the fact that humans can smell cancer in latter stages through the patients breath, but this isn't until 3-4. Dogs on the other hand, can sense it as early as stage 0 or 1-2 and they do this naturally. They can be trained to hone this skill and pick up on tin chemicals given off by cancerous tumors. Some have even been taught to alert diabetics to low blood sugar levels.
The video below from BBC Earth shows one particular dog, Max, who smelt cancer in his owner Maureen before any medical scans could pick it up.