Florentine Codex
<p>Smallpox / Plague book 12 pg. 83.
"But before the Spaniards had risen against us, first there came to be prevalent a great sickness, a plague [strike]. ...spread over the people a great destruction of men. At this time this plague prevailed indeed sixty days - sixty day-sings - when it ended, when it diminished; when it was realized, when there was reviving, the plague was already going toward Chalco. [...] at that time the Mexicans, the brave warriors were able to recover from the pestilence.
And when this had happened, then the Spaniards came."
The Florentine Codex, book 12, pg. 83ff, Aztec reports to Fray Bernardino de Sahagun, 1545-1590</p>