3The Bubonic Plague(s)
The bacterium, Yersinia pestis, has beaten the human race into a gibbering mess with such regularity that it's embarrassing. The first known outbreak--the Plague of Justinian--hit the Byzantine Empire in 541 AD. At its peak, the contagion killed an estimated 5,000 people a day in Constantinople.
Just look at these smug bastards.
Think of it this way--if you met a friend for a nice cup of Turkish coffee, chances were one of you was either A) infected or B) already dead. And if your buddy looked healthy, it was probably a good time to go update your will.
"Sigh. Stood up again."
After killing 50-60 percent of Europe's population, the Justinian Plague laid low for a couple centuries, repackaged itself as the Black Death and killed a third of Europe from 1348-1350. Like your mom's cooking, the bubonic plague wouldn't stay down--it hung around until the 1600s, when improved medicine and sanitation stanched its spread. And like your mom's cooking, the Black Death gave people fatal, necrotic tumors.
One hundred million people.
And because we love ruining your day, we'll inform you that the bubonic plague is still kicking about. Just move to the West Coast!
The reason you've heard of the Black Death but might not have heard of Spanish Flu, is because Black Death wound up with a much more menacing name. But make no mistake; the Spanish Flu made Black Death look like a pussy.
Spanish Flu first showed up in 1918. It was an H1N1 strain, a term normally associated with swine flu, which is Spanish Flu's underachieving younger brother. The 1918 outbreak infected a third of Earth's population (about 500 million people caught it) and it killed a third of Europe, putting it on even footing with the Black Death.
The similarities end there. Although both diseases killed roughly the same amount of people, the Black Death took about 200 years to do it.
Spanish Flu did it in two years.
Remember, when the Black Death made the rounds, most folks' idea of hygiene was throwing their feces out of second-story windows. The Spanish Flu was facing a much tougher crowd that understood quarantining, germ theory and antibiotics.
We're much more sophisticated these days.
That means if Spanish Flu had popped up during medieval times it would have had a very plausible chance at annihilating the human species altogether.
But it didn't. Probably thanks to wizards.
The flu did get some help from World War I. The sound of mortars blasting is like a dinner bell for pandemics. Thousands of malnourished soldiers crowded in cramped trenches were ideal circumstances for its rapid spread. Furthermore, neither the Allied nor Central Powers felt particularly inclined to report the Flu's spread due to a fear of appearing weak. The human race may as well have licked a pay phone at a bus station.