I suppose it is a sort of phobia, of amusment park rides.
Like most fears, fearing you will die on an amusment park ride is slightly irrational. You're more likely to die in a car crash. But I'm assuming you don't have a fear of car rides.
The thing to do is, pick rides that won't give you a heartattack, but don't let the fear of "Disney World Death" keep you from having a good time. I mean; it's Disney, not a test center. The employees try to keep the rides an park as safe as they can, not only for your benefit but their own as well. Thousands of people ride those rides every day, and we're not heading about daily death tolls at Disney world, so you have little to fear.
If worse comes to worse, and you're really against the rides, enjoy the park's other attractions. Have a good time, and don't let a bunch of freak accidents preven you from doing so.