Good observation. Ignorance of sanitation and hygiene was a significant cause of death. Diseases such as dysentery, a common cause of death among Civil War soldiers is virtually unheard of today. Even the rudimentary sanitation procedures enacted by Letterman slowed the spread of disease and enhanced survival rates.Deaths from disease impacted urban populations much more than rural ones so death rates varied by location quite a lot. You can see this with the disease issues both the US and CS armies had early in the war - soldiers from rural areas arrived in camp and got sick after being exposed to diseases they'd never seen in their small communities.