In expanding on what Old Fuff has written, sanitary conditions of the day weren't even close to what you have now. Consider just the bullets, balls or what-have-you themselves. Not considering pre-made ammo for the moment, most ammo was made in the field under the poorest of conditions, then stored in skin or leather pouches that were hap-hazardly tanned and cured. In addition, smearing rancid tallow on them for lube introduced all kinds of little beasties into the mix, followed by the fact that the personal hygiene of the shooters themselves usually left much to be desired at that time; so that, if the shot didn't kill you outright, you could easily die of infection of the wound. It's a fact that more people died of disease and infection during the Civil War then were killed out-right in the field.
Paper-patch bullets were particularly nasty as the lube soaked up all kinds of stuff while being carried in a belt. Kind of like having a Kimodo Dragon in a holster...