....The Civil War was America's bloodiest conflict. The unprecedented violence of battles such as Shiloh, Antietam, Stones River, and Gettysburg shocked citizens and international observers alike. Nearly as many men died in captivity during the Civil War as were killed in the whole of the Vietnam War. Hundreds of thousands died of disease. Roughly 2% of the population, an estimated 620,000 men, lost their lives in the line of duty. Taken as a percentage of today's population, the toll would have risen as high as 6 million souls.

The human cost of the Civil War was beyond anybody's expectations. The young nation experienced bloodshed of a magnitude that has not been equaled since by any other American conflict.

A "casualty" is a military person lost through death, wounds, injury, sickness, internment, capture, or through being missing in action. "Casualty" and "fatality" are not interchangeable terms--death is only one of the ways that a soldier can become a casualty. In practice, officers would usually be responsible for recording casualties that occurred within their commands. If a soldier was unable to perform basic duties due to one of the above conditions, the soldier would be considered a casualty. This means that one soldier could be marked as a casualty several times throughout the course of the war.

Most casualties and deaths in the Civil War were the result of non-combat-related disease. For every three soldiers killed in battle, five more died of disease. The primitive nature of Civil War medicine, both in its intellectual underpinnings and in its practice in the armies, meant that many wounds and illnesses were unnecessarily fatal.

Our modern conception of casualties includes those who have been psychologically damaged by warfare. This distinction did not exist during the Civil War. Soldiers suffering from what we would now recognize as post-traumatic stress disorder were uncatalogued and uncared for.

Information Link

https://www.civilwar.org/learn/articles/civil-war-casualties