There's interesting research suggesting that some fear of snakes is instinctual in monkeys, having their brains hard wired to react negatively to things that move with a slithering motion. Hard to say how much that applies to people though. Me, my husband, his cousins, and our daughter all love snakes. One of his uncles and his mom are terrified of them, but niether of them can point to any specific event or anything someone tried to teach them that made this happen. Paul (the uncle) has even tried to get over his fear by being around snakes; he helped a friend move a big retic to a new enclosure, he's held other peoples snakes, and even goes to pet shops to look at them behind glass, all to no avail. He can mitigate his reation, but he still gets bad heebee jeebees.

So, it might be a little of both and down to the individual.