Bob, you make very good points.
In my study of Camp White Sulphur Springs and the Hospital there, I come to realize that these Camp sickness were not so much the lack of immunity, because there were periodic outbreaks of such diseases as Yellow Fever in the lower south from time to time, but a change of lifestyle.
These men had been use to living in houses in small groups. And yes the immunity thing did play a part in these men gather together and becoming exposed to each other. But basically they were all healthy when they joined. The thing that broke their health down was exposure to the weather. The living outside for longer periods of time than they were use to without proper equipment such as tentage. Basically the inability of these men to manage their enviroment being hot, or wet from sweat, or rain, during the day, and cold or chilled during the nights.
The second part of this is that once a disease of any sort got started the men were not in a physical condition to fight them off because of fatigue due to marches and camp duties, which was quite different from farm and ranch living.
Another factor, which I also came to realize, is that even the most common diseases such as Measles were deadly. Why? It was because of complications of the above factors which lead to secondary diseases such as Pnuemonia. While diseases like Typhoid and Small Pox were deadly enough, they were respected and feared enough that these case were immediately isolated to prevent their spreading. Therefore they were not usually the main causes for large numbers of deaths.
One more factor that you hit upon was their medical practise. You mentioned the Calomel. This is true that Mercury based medicines such as Calomel and Blue Mass were widely used and prescribed for all sorts of illness during that time. Abraham Lincoln was known to carry "Blue Mass" on his person and take it daily for various ailments. And Mercury poisoning is one of the explaination for Lincoln change in appearents in photographs taken during the War. Mercury poisoning and opiate drug addictions were the main causes of "old Soldiers Disease" after the war. The point being is that Mercury poisioning did not act that fast as these men died, but did contribute to their overall inability to fight off the effects of diseases.
Fortunantely prescription medicines in the south quickly dried up and the southern doctor were more adapt in using herbal and homeopathic medicines to cure their patients. The homeopathic medicines took longer and the most effective medicine was usually time, but southern soldiers actually faired a little better because of that, than their northern counterparts with their "modern" medicines. Of course we have to factor into that, that the larger numbers of Northern soldiers from northeastern states who lived in big cities would have had the same problems with outdoor living as the farm boys of the south, and that may have played into that figure.