There were more than 150 prisons established and utilized by both sides during the Civil War. About 195,000 Northerners spent time in Confederate prison camps and about 215,000 Southern soldiers were in the Union prisons. The casualties were high as over 30,000 or 15.5% of Union soldiers died in Southern prisons. Andersonville in Georgia was the worst claiming the lives of about 13,000 or 29% of those who entered. On the other side nearly 26,000 or 12% of Confederate captives perished in Northern prisons. Elmira Prison in New York had the highest mortality rate with 25% or 2,963 deaths.
American Civil War page