I'd go with measles, which is not on the list.
Source: Elizabeth Whitley Roberson, In Care of Yellow River: The Complete Civil War Letters of Pvt. Eli Pinson Landers to His Mother (Gretna: Pelican Publishing Company, 1997), p. 36.“…we have witnessed the death of one of our fellow solgers to wit Thomas Sanders. He died with a relapse of the measels. He got most well of them and exposed hisself in the rain. His relapse was very hasty to death. He only lasted 5 days the last round. He died last night about 1 o’clock. It was a very solemn occasion. He was out of his senses all the time. I was detailed to wait on him 24 hours. It almost wearied me down for his was trying to skip off all the time. He said he was going home but the poor fellow will return home with his eyes closed. … It was heart rending to hear the bitter cries of him. The poor fellow called his Mother often. He died very hard indeed. … The sick sees hard times for they are lying in the hospital tents on some straw. God forbid that I shall ever spend my last days in such a place for it is awful to see the sick groaning…”
☹Diarrhea killed my ggguncle in a confederate prison camp.
Although it may sound strange to us today, Diarrhea, also known as the runs, Tennessee trots or Virginia Quickstep, was in fact the biggest killer in the war , killing close to 100,000 Union and Confederate soldiers.
The terms diarrhea and dysentery are often found together. Dysentery also called bloody flux simply means diarrhea with the presence of blood and mucous. Dysentery was typically caused by bacteria or sometimes amoeba parasites.