Chicago, Illinois. Camp Douglas the "accidental prison" often referred to as the "NORTH'S ANDERSONVILLE, and "80 Acres of Hell", between 1862 and 1865, 6,000+ Confederate POWs dead, and 1500+, UNACCOUNTED FOR.
Even the Chicago population tried to help these poor prisoners. It was a hell on earth at Camp Douglas, that not even Andersonville could out do, if one only looks at the ratios. In 1864 alone, 1162 prisoners died. In all, almost one third of all the prisoners died, of either scurvy, or small pox.