The idea that a cancer or a virus would have a smell? Is not intuitive. What happens as it changes your bodies methabolic pathways, of which you have thousands going on in every cell? There's different things happening. So why can the dog generalize for the same reaso, you can generalize the gate. You can track my footprints if i teach you what footprints look like on the wet sand and how they are distributed. And then it extrapolates and says, ah, this smells like the stuff they used to want me to find.