I am not doubting the above and don't take offence but, reciently read,"Portals to Hell", Speers,he might not agree.
Basic overview Camp Morton it was originally a fairgrounds created in 1852. At the outbreak of the war in 1861 Gov.Oliver P.Morton designated the fairgrounds a recruiting camp.Many Ind. Regt. were trained there,perhaps the 15IND..

Camp Morton,the prison, recieving its first prisoners Feb.24,1862 from Ft.'s Henry & Donelson.Speers states the number around 652 men mostly men sick or wounded of KY.,TN.,ALA. regiments. He states the original gaurds were the 60IND.Inf..
With the signing of the Dix-Hill Cartel in July of 1862 the Camp was emptied Sept.of that year.
The prison was reactivated in Feb. of 1863 and by Oct. held 2,362 prisoners including 30 Civilians. Dec. of 1863 held 3,300 at this time it was guarded by 234 men of the 71IND.Inf.though 504 men of the regiment were on parole at the camp totaling 738 and two companies of the 63IND.Inf.in simular situation.
Other Regiments that saw duty at Camp Morton were the 51IND.Inf. and the 73IND.Inf. both having men reciently exchanged but, whos officers were still held at Richmond.These enlisted men in a kind of no mans land so the Government put them to work repairing and gaurding the camp.
The camp at one time was run by the 5th Invalid Corps.as its sister Camp Chase,Ohio.
The total number of Confederate men held during the entire life of the camp was 12,082 and had a 15 % death rate or 1763 deaths.Of the Northwestern prisons Camp Morton was average with total prisoner populations simulair to Johnson Island,12,000 ,Elmira,12,123, and Rock Island,12,409.The largest being Camp Douglas with 30,000.(NPS)
Few understand the system of parole and exchanges as it doesn't make sence by todays thinking. Like most northern camps ,Camp Morton, also held men on parole ,Federal regiments,but not yet exchanged. This might be the 15IND. connection.I am not sure purhaps I can be enlightened.