*Anything*, hum. You've just not met it yet.
Modern society protects most people from *real* fear fairly well. I didn't experience real fear until I was 30 during Gulf War 1. This was in a makeshift and well ventilated air raid shelter, dressed in shorts and T shirt, surrounded by RAF personnel in full NBC kit (and very frightened eyes through the respirators) as Mr Hussein was "test fired" missiles possibly carrying chemical warheads...