I'm no expert on it but my vet does a lot of work with it and from what I remember him saying and what I've read it sounds to me like it is unknown in the wild. Of course it came from somewhere but we just don't know where. So imports are not a particularly high risk for IBD, at least before they pass through dirty importer facilities and pet stores.
However, my vet did say something about a study of boas he was involved with where 50% of them had it. I don't know how random his sample was (i.e. was it only from collections with symptoms etc.). Boas can apparently live with it long enough to spread it through breeding to mates and offspring however he blamed lax mite control for the worldwide spread of the disease in captive boa populations. There was some speculation early on that it might be spread airborne and I'm not sure that that has been eliminated but I think it was my vet who used the AIDS analogy indicating that it probably spreads through fluid contact when boas mate, give live birth, or via mite blood transfer.
My understanding is that a ball python would lot live long with the disease at all so there is probably much less risk of ball pythons passing it between each other as long as you don't have mites and do a few months quarantine. The big risk is if you get ball pythons from mite-infested locations that have boas (pet stores, lax importers/wholesalers, private keepers with both animals and mites).
By the way, mites come from Asia so ball pythons from Africa should not have them in the wild and I believe most respectable importers and wholesalers are now working hard to eliminate mites that might come in with their Asian imports.