Fowl cholera or Pasteurella multicida is a bacterial disease that thrives in unsanitary conditions and can survive in soil to reinfect avian species. It causes chronic pain and suffering in the chicken with possible death. Fowl cholera can be spread by rodents and possibly by feral cats and pigs.
Caged hens are relatively easy to keep clean and they don't have access to soil. Should a hen become infected then disease spread through the shed should be slow as each hen has limited access to the rest of the flock unlike a large free flock where there is the possibility for rapid spread of any bacterial infection.
New vaccinations are available that can provide some protection.
The best protection in my opinion is to keep the hens off the ground, clean out the shed between each batch and have a rodent control plan in place.