According to a letter of Marshal Macdonald a normal sickness rate in the army was considered 1/10 in peacetime and 1/7 in time of war. That's sickness not mortality though

Susan

During the US Civil War, the United States army numbered around 2,500,000 overall. Combat losses totaled around 110,000; deaths from disease totaled around 250,000.

As for POW conditions - the US POW camp at Elmira, New York had a mortality rate around 25%; the confederate POW camp at Andersonville, Georgia had a rate of around 29%.