I had a bad experience as a child and it stayed with me.
In the Army we had a training session where six students and an instructor sat in a screened-in bleacher with a sandpit containing about five snakes. Of course I was in the front row. Being within two feet of the snakes was almost intolerable; I made it through but my heart was pumping out of my chest. That's when I realized I had the fear. Still do.