Short answers, pulled from the story.
About 56,000 soldiers died in Civil War prison camps, accounting for nearly 10% of all Civil War fatalities. Of the 211,411 Union soldiers captured, 30,218 died in captivity; of the 462,684 Confederate soldiers captured, 25,976 died.
The exchange system collapsed in 1863 when the Confederacy refused to treat captured Black Union soldiers the same as white prisoners, arguing they were likely former enslaved people. This made reciprocal exchange impossible, and Gen. Ulysses S. Grant later opposed resuming large-scale exchanges because the prisoner gap gave the Union a military advantage.
Andersonville, formally Camp Sumter near Andersonville, Georgia, had a death rate of 28%. Over 14 months, 13,000 of the 45,000 Union soldiers confined there died, making it the deadliest Civil War prison.
The Dix-Hill Cartel was a July 1862 agreement negotiated by Union Maj. Gen. John A. Dix and Confederate Maj. Gen. D. H. Hill. It established a scale of equivalents for exchanging prisoners - a colonel or navy captain was worth fifteen privates - and required authorities to parole any prisoner not formally exchanged within ten days of capture.
Just over 5.6% of captives in Northern prisons died, compared to 14.29% in Southern prisons. Elmira Prison in New York had a death rate of 25%, and Camp Douglas in Chicago recorded roughly 15% over four years, while Andersonville in Georgia reached 28%.
Galvanized Yankees were Confederate prisoners of war who agreed to join the Union Army rather than remain in captivity. Around 5,600 Confederates took this option and were stationed in the West, where they served against Native American adversaries rather than former Confederate comrades.