Obviously, I just read the whole thread of 18-pages, thus all the comments and questions from posts pages ago.
No one has stated that White Leghorns have disease resistance. If I can locate a quote I found about genetics long ago I will come back and post a link. The gist of the story is that WL flock was more resistant to Marek's than RIR flock in a university study. It was one of those midnight googles from some months past.
Edited: Not the earlier article I referenced, but one that describes resistance to Marek's in White Leghorns:
http://ps.fass.org/content/80/8/1064.full.pdf
Different breeds appear to have differing levels of resistance to different diseases and parasites.