I'm looking to expand my flock with a few different breeds of dual purpose birds, I would like to add a few RIR's, Buff Orpingtons and Wyandotte...Looking for some fertile eggs to incubate. Willing to pay but not what they are asking by the breeders and don't want to risk shipping.

it's a slow painful death that is held off by the host organism's own immune system or antibiotics. Incurable. In rats, most of the population has it, (thanks crappy humans and your crappy thoughtless breeding programs!) and the rats with stronger immune systems will be carriers, but asymptomatic usually. Not something you want a healthy flock infected with, though I'm not sure if it is or isn't curable in chickens. My guess from the rats version is "no". (I know it won't be exactly the same between each species variation, so the rats might to be the prime example of how it affects chickens.)