The Alton Military Prison was a prison located in Alton, Illinois. The prison opened in 1833 as the Illinois State Prison and originally held 24 cells. By 1857, when it was replaced by a new state prison in Joliet, the prison housed 256 cells. In 1862, the U.S. government reopened the prison to house Confederate prisoners of war during the Civil War. The prison housed over 11,000 prisoners during the war. Deaths at the prison were more common than at other Union prisons, and prisoners faced harsh conditions and regular outbreaks of diseases such as smallpox and rubella. 1,534 Confederate soldiers and many Union soldiers and civilians are known to have died at the prison.