5 times parasites affected the course of wars

Some of the most consequential actors in past wars weren't the soldiers storming the ramparts, the generals issuing orders, or even engineers who designed the weapons and armor necessary to win. Sometimes, wars were decided by much smaller combatants: bugs and worms.

Before the rapid increase in medical knowledge around World War II, parasites caused epidemics that claimed entire formations and tipped campaigns forcefully for one side to the other.

Both British and American forces in the Revolutionary War scheduled offensives in the South around "the sickly season," the hot summer months where mosquitoes and the diseases they spread were likely to claim hundreds or thousands of lives in an army on the march.

Both sides suffered when they forgot their lessons, but the worst afflicted was probably British forces in the Carolinas in 1780-1781. The British captured Charleston before the sickly season set in but was ravaged in the months following. Senior officers, doctors, and thousands of soldiers were infected with malaria and yellow fever among other diseases and entire formations became combat ineffective.

This slowed the progress of the British Army and forced it to fight at times it otherwise would not have. Patriots, who were less affected, capitalized by forcing battles when the British were sick and claiming victory when the British pulled out of diseased areas. By mid-1781, the generals were done fighting mosquitoes in the South and moved most of their forces north to Virginia.

The Russian winter and the burning of Moscow get a lot of credit for destroying Napoleon Bonaparte's army, but top honors should probably go to the Polish summer and the legions of lice it created in 1812. Lice secrete the typhus germ in their feces, and typhus causes severe fever, vomiting, and death.

When Napoleon and his 680,000 men went on the march into Russia, they had to cross through Poland and its lice epidemic. It was there that his men began to catch typhus which spread through the ranks. The disease and the resulting desertions as soldiers fled the infected camps took away half of Napoleon's fighting force before he fought any major battles with the Russian Army.

Before Napoleon's Grande Armée fell to lice, a smaller force sent to Haiti – then known as Saint-Domingue – was destroyed by yellow fever carried by mosquitoes. The emperor sent 33,000 men to put down the government of Toussaint Louverture in Haiti and turn the island back into a French slave colony.

Unfortunately, the Haitians they would fight were largely inoculated to yellow fever, but the French were not. France initially gained the upper hand by recruiting support from factions in the Haitian forces. But the French lost 90 percent of their men to mosquitoes and the disease. When their local allies learned about the re-introduction of slavery and turned on the French, Napoleon recalled the survivors.

Southerners before the Civil War had a reputation for being dimwitted and lazy, something doctors later found out was due to the epidemic levels of hookworm that existed there. Hookworms invade through bare feet or fingernail beds and spread throughout the human body where they can live for five years, causing malnutrition and exhaustion while slowing brain function.

While both Northern and Southern armies caught the parasites during the war, the South had it worse. And, the Confederates had to recruit more deeply from a population that had been afflicted with worms in their youth. People who caught the worms during developmental periods often suffered from developmental defects like stunted growth or mental deficiencies.

Seriously, body lice are just the worst. In World War I, every army had to deal with constant lice infestations. But most were lucky in that only more mild diseases like trench fever were spread. Typhus struck Serbia in 1914 though, and the disease spread across the Eastern Front.

At least 20,000,000 Russian soldiers contracted the disease and half of them died. The Germans refused to invade some areas to prevent the disease spreading to their ranks and so were able to avoid their own epidemic. The Russian Army eventually dissolved and Germany was able to send troops west, prolonging the war.

In World War II, recognizing the risks of an outbreak among their troops, Germany forced a Jewish doctor to create a vaccine for typhus.