A State marker designates the remnants of a portion of a cellblock of the Alton Federal Prison as the site of the First Illinois Prison, built in 1831. During the Civil War, the prison reopened as a military detention camp because of overcrowding in the two St. Louis prisons that housed Confederate prisoners of war. A smallpox epidemic in December, 1862 killed as many as 2,200 prisoners.
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Approximately 300 prisoners and Union soldiers who died of smallpox were buried on a nearby island (once called Sunflower Island and currently under water) where a quarantine was set up. Those who were not buried on the island were interred in a special plot in North Alton, known today as the Confederate Soldiers' Cemetery.
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