The same question I'm answering now, I was asking myself as I ingested a ghost chili pepper a few hours ago. the pungency our tongues interpret as spiciness comes from a chemical compound known as capsaicin. This chemical is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which in comes into contact with. The Scoville scale is the measurement of capsaicin concentration.
The ghost chili pepper holds the 1st prize for the highest on this scale coming in at one million SHU's. However due to human tinkering chili peppers have been grafted together to create hybrids. These hybrids such as the Carolina reaper(which is 1.5x hotter than the ghost chili pepper.), blow the Ghost Chili pepper out of the water. But the ghost chili pepper remains the hottest natural chili pepper known to man.