Prisoner-of-war records have not been destroyed and are excellent sources for high death rates at each of the different prison camps. Records were kept for accounting and management purposes. The idea that anyone outside the military would review records for humanitarian or other such reasons would never have occurred to the clerks keeping prison camp papers.
If you know a Confederate soldier's name and unit, and if he actually spent time in a Northern prison, it should be fairly easy to locate his POW records. Let us know if we can help!