#2. Spanish Flu
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.
#1. The Toba Event
Around 75,000 years ago, something very odd happened. Genetic diversity imploded, the population bottlenecked and suddenly most species of Homo whathaveyou vanished like the daughter from the first season of Family Matters.
The prevailing explanation is the Toba catastrophe theory, which posits that a supervolcano erupted near Lake Toba in Sumatra. The Toba eruption created an explosion of around 2,000 megatons (more than 150,000 Hiroshima bombs). That's the sort of thing Michael Bay would shoot if you locked him in a windowless box for a year and then gave him a bucket of meth and the GDP of Australia.
The cloud of ash pulled the same trick as the K-T disaster, obscuring the sun and plunging the planet into a volcanic winter. The global temperature dropped up to 10 degrees, which is mind-boggling given that climatologists lose their shit over one and two degree shifts.
Oh, and before you consider nuking Yellowstone National Park to fix global warming, let's consider the effects of the eruption on the human species.
The Toba blast is the reason Homo erectus isn't at your family reunion--the ensuing havoc resulted in the extinction of every member of the Homo genus save Neanderthals and modern man. Homo sapiens didn't escape unscathed, though--the blast may have reduced the human population to approximately 10,000 people.
Ten. Thousand. People. In comparison, Wrigley Field holds around 40,000 fans. After the Toba event, the entirety of mankind, all the remaining humans in all the world, could be consigned to the bleacher section.
Ten thousand. Wrap your head around that figure. Some geneticists even speculate that the population dropped to 5,000--a figure roughly equal to the seating capacity of 12 747s. The Toba event was the closest the human species has ever come to extinction, and we had yet to move past relieving our bowels in public. It would have been an ignominious way to go.
And how did humans rebound from such a supervolcanic drubbing? The same way we got through all of these apocalypses: good old-fashioned fucking.
Saving the human race since 73,000 BC.
For more on the apocalypse, check out The 6 Best 2012 Apocalypse Theories (Are All Bullshit). Or find out what's worse than going extinct, in 6 Formerly Kickass Creatures Ruined by Evolution.
Or stop by Cracked.com's Top Picks to see why Brockway might bring about the next plague (it's because he doesn't wipe).