...on the night when the Aztecs drove Cortez and his men out of Mexico City, killing many of them, an epidemic of smallpox was raging in the city. The man who organized the assualt on the Spaniards was among those who died on that noche trista, as the Spaniards later called it. The paralysing effect of a lethal epidemic goes far to explain why the Aztecs did not pursue the defeated and demoralized Spaniards, giving them time and opprtunity to rest and regroup, gather Indian allies and set siege to the city, and so achieve their eventual victory.

William H. McNeill
Plagues and Peoples