Breeding for resistance should be done responsibly and with an idea of which diseases can (or should) be bred with a resistance towards. Hardy birds should also be kept in good health to adequately gauge thier response to the pathogen, ideally.
Just as an example-
I think Mareks and endemic/mild Newcastle (NOT Exotic) could be bred for resistance.
Pullorum-Typhoid should not be allowed in a flock.
Although we are still learning, one good information on this subject is in The Chicken Health Handbook"
and if you really want to get technical, on the web, such as looking at potential projects-
Edited by birdsNbeesNseeds - 9/17/10 at 6:19am