From Fray Bernardino de Sahagún, Florentine Codex, Book 12, Chapter 29 (Mexica)

Here it is told how, at the time the Spaniards left Mexico, there came an illness of pustules of which many Mexicas died; it was called "the great rash" [smallpox].

[Even] before the Spaniards appeared to us, an illness broke out, a sickness of pustules. It began in Tepeilhuitl, and it spread over the people as a great destruction. Large bumps spread on people; some were entirely covered--on the face, the head, the chest, etc. [The disease] brought great desolation; a great many [people] died from it. 

[People] with the illness could not walk, they could only lay in their dwellings and sleeping places. They could not move; they could not stir; they could not change position, nor lie on one side, nor face down, nor on their backs. And when they stirred, they screamed. The pustules that covered people caused great desolation; a great many people died of them, and many just died of hunger; [for] no one took care of others any longer.

On some people, the pustules were widely separated, and they did not suffer greatly, nor did many of them die of it. But many people's faces were marred by it; their faces or noses were pitted. Some lost their eyes; some were blinded.

At this time, the pestilence lasted sixty days, sixty fateful days. It began in Cuatlan, where it became so well established that nothing could stop it, and it spread in the direction of Chalco. And many were disabled or paralyzed by it, but they were not disabled forever. It broke out in Teotleco, and it abated in Panquetzaliztli. The Mexicas were greatly weakened by it.

And after this, the Spaniards came; they marched from Tezcoco. They appeared from the direction of Quauhtitlan and stopped at Tlacopan. There, they gave out assignments and divided themselves. Pedro de Alvarado was made responsible for the road coming to Tlatelolco, and the Marquis [Cortés] went to Coyoacan. Here, he took charge of the Acachinanco [road], which led to Tenochtitlan. The Marquis considered that those of Tenochtitlan were valiant warriors.

And at Nextlatilco, or Iliacac, the war first began. The [the Spaniards] quickly reached Nonoalco, with the [Mexica] warriors pursuing them. None of the Mexicas died; then the Spaniards turned back. The warriors fought in boats; the warboatmen threw darts at [the Spaniards]. Then [the Mexicas] entered [Nonoalco]. Subsequently, the Marquis sent [his soldiers] toward Tenochtitlan, following the Acachinanco [road]. Many times they fought, and the Mexicas confronted him.