First of all a lot of times, CSA casualties were under reported. One example is from the battle of Cedar Mountain in Aug 1862, and I think Jackson reported 3 missing. this after an all day battle where several brigades were routed. I think a similar example is at South Mountain, Sept '62. CSA casaulties usually listed at about 1600 but they had units like Ripley's brigade that seems to have been driven down the other side of the mountain and lots of troops must have gone missing. Because Ripley's troops really dont show up at the battle of Antietam 2 days later. In the 7 Days campaign, CSA numbers seem to be under reported; there is a fellow name of Dave Powell who has done a lot of research on that campaign and I think this is published somewhere.

Numbers for the 1864 battles of Spotsylvania and Wilderness seem awfully suspicious to me. At Wilderness both sides were fighting in close, the tactics were similar, both sides dug in, both had similar weapons, and similar quality of soldiers. There was no great flanking maneuvers or some some such and yet some reports give the south as losing merely 9,000 vs about double for the north. It is suspicious and there is now way to figure it out systematically, the under reporting is probably there and hard to really say how much.

CSA wounded lists were also done in a different way; I think they only listed those disabled by wounds. In any event there seems to be a different reporting system. As an example you can look at the numbers Douglas S Freeman (Lee's Lieutenants) gives for Gettysburg losses and I think it amounts to 15,000 wounded and 20,500 total. You can look at other sources you will see higher numbers, usually 28,000+. I myself have seen union reports indicate they captured 7,000 wounded and 6,000 unwounded soldiers, which if they had 4000 KIA, and say 20,000 wounded, the actual number of casaulties at Gettysburg might reach 30,000. Chickamauga is another battle where this time the south was mainly on the offensive and report about 20,000 vs 18,500 union, the numbers could be higher. Island Number 10; March '62 CSA captured differ significantly from union reports, probably owing to last minute reinforcement of the garrison. These are just some examples.

Oftentimes there is a 5:1 ratio of wounded to killed in action in these battles and sometimes you can see the numbers do not match up. Other times, you have to account for circumstances, if one side had to leave a battlefield quickly perhaps more numbers will be reported missing. So its' not a firm rule, just a guide.

BUt not to overemphasize, yes the union did lose more troops. Primarily in my opinion because the union was generally on the offensive, and they were generally in enemy land; where the south could defend their position and often have advance word of when/where the union troops were approaching. The south had several advantages in most battles because of this. Just look at how inept the south looks when they go on the offensive, Gettysburg is Army of No Virginia's first large scale defeat in about year. Antietam, while tactically a draw could have turned into a disaster if MacClellan had acted quickly as Lee's troops were dispersed and ripe for picking. Southern armies operated no better than northern when they went on the offensive in enemy territory.

Finally, the issue of disease in camps always comes up; and in fact the union had many more troops in the ranks than the south. Numbers are hard to estimate, I would estimate about 850,000 southern troops served for significant amount of time. Maybe 2 million northern troops. So death by illness would have to be larger on the northern side. That is about the best ideas I can give you.