enemies unseen..................

But these boys were also volunteers.  They wanted to become soldiers, and this was the only process being offered to move them around and get them deployed and shooting.  So they drilled and continued to drill until they began to feel the power of the whole, which was the entire point.

     It was also a big problem.  Most of these boys had arrived in what we would call good shape: lean and physically fit from lives on farms or strenuous work elsewhere, used to walking and being outdoors.  But they were weak in ways they never suspected and living together—sleeping in twelve-man Sibley tents and eating in mass messes—almost immediately reveal their lack of immunity to measles, mumps, and tonsillitis, childhood diseases that soldier from cities would have already experienced.

     These put them down in huge numbers, half or more during their first months of service.  So many were sick that Sergeant Spotts of the Eighty-seventh Indiana was shocked to see "only 400 guns (out of 1,000) stacked at night.  Though most recovered, many were weakened for the arrival of real killers, smallpox, and the streptococcus erysipelas.  Before long. boys were helping to bury their friends and neighbors, casualties of an enemy they couldn't see and hadn't signed up to fight. 

-Robert L. O'Connell, Fierce Patriot: The Tangled Lives of William Tecumseh Sherman