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It's July 1st, 1520 in the Aztec city of Tenochtitlan, Mexico, and the battle is underway.

The nobleman Kletlauac is one of tens of thousands of Aztecs who have taken up arms against Spanish invaders for trying to conquer their city. The Spanish conquistador Hernan Cortez arrived on these shores

with his soldiers about a year ago. At first, the Aztecs hoped to befriend the Europeans.

They welcomed them as their guests, showering Cortez with gifts of gold and letting him set up camp in their main

city. But Cortez did not come in peace. Within months, his soldiers started murdering Aztec

priests and chiefs in an attempt to secure power. Quetloak's brother, the Aztec emperor, was among

the many who were slain. Now, Quetloac and his fellow citizens are hungry for revenge.

They drive the Spanish to the city's edge in a series of violent skirmishes, and as the last of

the departing Spaniards disappear into the distance, Quetloak stops to catch his breath. He looks around

at the streets littered with the bodies of the Spanish.

His sword is sticky with blood, and his arm aches from the fight.

But he has strength enough to rattle his shields in triumph.

The Aztecs have done it, and the Spanish are gone.

Though defeated in battle, these Spanish invaders have inadvertently left behind an enemy more

powerful than any army. When Cortez and his men return in less than a year, Quetloac and 40%

of the city's inhabitants will be dead, and the surviving Aztec people will be too weak to mount a

defense. They will have been struck down by a deadly virus now known as smallpox. Millions more will be too weak to mount a defense. They will have been struck down by a deadly virus now known as smallpox.

Millions more will be killed by this disease in Mexico and far beyond.

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