Know Your Enemies!
1. The tick! - Ticks encountered in Europe transmitted Rickettsia, Lyme Disease (different from US Lyme Disease and rare), Tick-Borne Meningoencephalitis, Tick-Borne Relapsing Fever, and Crimean-Congo hemorrhagic fever. In the Pacific, a hemorrhagic fever related to yellow fever could be contracted.
2. The body louse! - TYPHUS! Death from typhus! Epidemic typhus occurs almost every time there is mass civil unrest, wartime conditions, or natural disasters forcing people together, even though endemic typhus no longer exists. The body louse can also cause “trench fever” and louse-borne relapsing fever.
You HAVE to get rid of (or at least control) these guys if you want to win any war. Otherwise you’ll have incapacitated or dead soldiers before you even set foot on the battlefield. Getting rid of the body louse (and typhus) was especially important.
As the German army invaded Russia during WWII, the multitude of problems they already had was compounded by the fact that they kept getting waves of deaths due to typhus. When Napoleon retreated from Moscow in 1812, more soldiers died of typhus than were killed in combat. And, of course, millions died of typhus in the Nazi concentration camps, including Anne Frank, and her sister Margot.
Posters from the Otis Archives of the National Museum of Medicine.