There seems to be some debate over this. According to Dr. Scott Stahl when he was on Reptile Radio, snakes do not get internal parasites from feeders (at least the type of parasites that would live on feeders - however, I'm sure it's possible if parasites get on a prey item from an infected animal). I've fed live to all of mine for the past 4 and a half year, and none of them have gotten internal parasites.
Parasites are very species specific. Warm blooded specific parasites don't survive in cold blooded animals.