There is a radio program out there (sorry, I don't have the link) featuring Dr. Scott Stahl, DVM. He is supposed to be one of the top herp vets in the US, at least according to the show's announcers. He talked extensively and in detail about RI's causes, and secondary infections, the proper technique for getting a culture, etc.
Very informative.
Worth doing a search for.
My take-away (may not be accurate) was that Retics seem to have more problems with RI's than Balls. He said that Retics tend to be in proportionally smaller confinements, and due the bulk and difficulty of moving them, many keepers don't get them out enough for them to really move around.