I agree about the rational/irrational description. For example, I have a rational fear of rather vicious, aggressive dogs (hasn't everyone?) but I do love friendly dogs. My fear of UK spiders was very irrational, as they aren't venomous.
The strange thing is, I saw quite a few dodgy snakes in Senegal, but didn't fear them, I just made sure I kept away. They were rather beautiful actually. Ditto scorpions, I saw many of those there too, but didn't screech or run!
I've always been ashamed of my arachnophobia. My naughty pupils used to put a big plastic spider in my register drawer, and I'd screech the place down, much to their great amusement.
I do hope I'm now 'cured' after our visit to BugzUK in Nowhere Lane.