Thanks for sharing that information Jean. There is another way to deal with CRD, and that is to breed for genetic resistance. The late Fred Jeffrey, the finest secretary the ABA ever had, was quoted as saying the most useful item in his medicine cabinet was an axe. Clean, airy coops, a proper diet, adequate parasite control, prevention of overcrowding, all contribute to a low stress environment which helps prevent disease symptoms from getting started.
That is the route that many have chosen due to the impracticality of securing and maintaining a germ free flock.