Well, I don't know about historical accuracy or realism, but I'm going to go ahead and say it:

Aliens.

Imagine a scenario similar to what happened to the Aztecs. An alien race, hell bent on conquest, invades the earth with their all powerful spaceships, whose mere presence destroys all our electronics via an almost aura-like EMP field. The humans try to fight back, and actually manage to repel the overtly-arrogant aliens, who were actually here without military sanction!

And then someone catches Andromediac Flu (Or something, but you get the idea.)

The disease is incredibly virulent, and does not differentiate between race or gender or sexual orientation. It is a terrifying disease that provides a slow agonizing death. The governments rush to contain it, but the alien invasion has left them undermanned and underequipped.

Eventually, the plague subsides. All the survivors are now immune to its effects, but the damage has been done. A drastic drop in life expectancy and a terrible lingering paranoia that will damage the psyche of all mankind for generations to come.

TLDR: Alien space flu.

Edit: Just got home and realized I crafted a reverse War Of The Worlds storyline. Well, it makes sense that they would've been immunized to something we weren't anyway.

Well, I don't know about historical accuracy or realism, but I'm going to go ahead and say it:

Aliens.

Imagine a scenario similar to what happened to the Aztecs. An alien race, hell bent on conquest, invades the earth with their all powerful spaceships, whose mere presence destroys all our electronics via an almost aura-like EMP field. The humans try to fight back, and actually manage to repel the overtly-arrogant aliens, who were actually here without military sanction!

And then someone catches Andromediac Flu (Or something, but you get the idea.)

The disease is incredibly virulent, and does not differentiate between race or gender or sexual orientation. It is a terrifying disease that provides a slow agonizing death. The governments rush to contain it, but the alien invasion has left them undermanned and underequipped.

Eventually, the plague subsides. All the survivors are now immune to its effects, but the damage has been done. A drastic drop in life expectancy and a terrible lingering paranoia that will damage the psyche of all mankind for generations to come.

TLDR: Alien space flu.

Edit: Just got home and realized I crafted a reverse War Of The Worlds storyline. Well, it makes sense that they would've been immunized to something we weren't anyway.

Well okay, from earth then.

Presenting 'The most delicious food in the world!', a vegan-approved drug that makes everything you eat taste 10 times better! Even the humblest of fried foods will blow your mind! This drug creates a boom in the food industry, and everyone is now on their quest to find better and tastier foods!

Then it all goes downhill.

People grow fat. Not just any normal kind of fat, but too fat to go about their business as normal. Infrastructures crumble as the people who used to run it can no longer get to work on time, if at all. Even the chefs who used to cook the food that caused this problem can hardly continue. Mass diabetes afflicts everyone, and people begin to die. There is no escape.

Eventually, the last healthy man looks down upon the empty streets filled with food wrappers and abandoned fast food stores, and realizes that he has perhaps witnessed the fall of humanity as we know it.

TLDR: Wall-E without access to space flight

Well, I don't know about historical accuracy or realism, but I'm going to go ahead and say it:

Aliens.

Imagine a scenario similar to what happened to the Aztecs. An alien race, hell bent on conquest, invades the earth with their all powerful spaceships, whose mere presence destroys all our electronics via an almost aura-like EMP field. The humans try to fight back, and actually manage to repel the overtly-arrogant aliens, who were actually here without military sanction!

And then someone catches Andromediac Flu (Or something, but you get the idea.)

The disease is incredibly virulent, and does not differentiate between race or gender or sexual orientation. It is a terrifying disease that provides a slow agonizing death. The governments rush to contain it, but the alien invasion has left them undermanned and underequipped.

Eventually, the plague subsides. All the survivors are now immune to its effects, but the damage has been done. A drastic drop in life expectancy and a terrible lingering paranoia that will damage the psyche of all mankind for generations to come.

TLDR: Alien space flu.

Well, I don't know about historical accuracy or realism, but I'm going to go ahead and say it:

Aliens.

Imagine a scenario similar to what happened to the Aztecs. An alien race, hell bent on conquest, invades the earth with their all powerful spaceships, whose mere presence destroys all our electronics via an almost aura-like EMP field. The humans try to fight back, and actually manage to repel the overtly-arrogant aliens, who were actually here without military sanction!

And then someone catches Andromediac Flu (Or something, but you get the idea.)

The disease is incredibly virulent, and does not differentiate between race or gender or sexual orientation. It is a terrifying disease that provides a slow agonizing death. The governments rush to contain it, but the alien invasion has left them undermanned and underequipped.

Eventually, the plague subsides. All the survivors are now immune to its effects, but the damage has been done. A drastic drop in life expectancy and a terrible lingering paranoia that will damage the psyche of all mankind for generations to come.

TLDR: Alien space flu.

Edit: Just got home and realized I crafted a reverse War Of The Worlds storyline. Well, it makes sense that they would've been immunized to something we weren't anyway.