Along the Western Front in 1915 soldiers began incurring a previously unknown infectious disease that was originally thought to be an enteric fever. No records were kept (except by the Americans in 1918), but it is thought that the number of allied soldiers thus affected was about 500,000. The soldiers themselves called this disease “Trench Fever”. It was debilitating but the vast majority of those afflicted recovered in a few weeks, and deaths were extremely rare. However, there sometimes were lingering effects, for example, the famous British fantasy writer J.R.R. Tolkien caught Trench Fever in October 1916 and was never returned to full duty. You can read more about Trench Fever by clicking here.
Recently doctors at the University of Colorado found four cases of Trench Fever among the homeless population of Denver. You can read about this by clicking here.