Do you know if there is an ethnic/racial profile of susceptibility? Or is that such a politically incorrect question that researchers haven't been allowed to study it or publish study results?
I don't think there is much ethnic difference here. It is more likely to occur in crowded (generally poor) communities where many infections spread better and in which it had already been present. I think of India, Nigeria, Brazil and SE Asia as the main remaining trouble spots. Central America has some, but not as much as the those. Medieval Europe had a lot of it so Caucasians certainly were susceptible then. I wouldn't expect much resistance to evolve since then to such a slow killer, unless it was as a side benefit to bubonic plague resistance. Historically some long isolated populations (e.g. Hawaii) were less resistant. There actually has been quite a bit of research on how the immune system handles or fails to handle leprosy, but I'm not up to speed on it. I think the 3% susceptible figure came from epidemiological models of its spread.