It is more complicated then poor treatment or puposefull poor treatment.I would of rathered to be captured in late 1862 early 1863 then latter in the war.If you have access to O.R. Series II Vol.V.Corespondence on Prisoners of War and State, tracks the movement of theses men.First, corespondence between two prisons.Feb.2, 1863,telling of prisoners being sent," Many are sick and nearly all poorly clothed and without blankets." A southern man cannot imagine the bitter cold of a northern winter.Mean temperature for Dec.,Feb.1863 in the mid to high twenties.Barracks made from green lumber couldn't keep the wind out.Poor organization rampant.In a report of Feb.7 officer stated the rebel prisoners suffered from the severe cold.
You might also realize many died or became deathly sick in transport having only what they were captured with to keep warm.Often only a blood soaked rags with no shoes.I think they were treated well for the circumstances which were bad.Most captives held at Camp Butler,from Arkansas Post, were from Regiments in and around Texas. Of those most were Foriegn Nationals Germans, Poles and Irish over half wanting to take the Oath of Allegience.
I don't see Small pox there but, was a problem at Alton,IL. around the time in question.
I have two-fold interest. The 127IL.Inf.( my GGuncles )guarded some from Arkansas Post to Camp Douglas. In May of 1863 my GGGrandfather was transfered to Springfield,Camp Butler and Camp Yates, for recruiting.These camps both originally were designed as Muster/training camps.
I agree with the other posts looks comparable.These deaths are over a period of time.Attrition is your word.Admire these men! My guess men of honor. The half that refussed the Oath.For which time had no mercy.