It sounds like the treatment of prisoners was very much dependent on the whims and attitudes of whatever ranking officer was in charge at any given time. Another problem was the lack of resources in the south.
In Chicago (Fort Douglas?) was a prison camp that started out being extremely lax with confederate prisoners wandering the streets of Chicago. Over time the place became extremely harsh due to the rising prison population and attitudes of the commanding officer.
Camp Douglas. Bad sanitation at these places made for a high mortaility rate.