Although some of the Native Americans believed the Europeans caused the disease that killed half their tribe and 80% of the Europeans, de Vaca's host persuaded them not to kill what was left of the foreigners for revenge, reasoning that A. the Europeans were too sickly to sacrifice to their gods.
B. if the foreigners had the power to cause the disease, so many of them wouldn't also have died.
C. more Europeans might show up and seek revenge against the Native Americans.

B. if the foreigners had the power to cause the disease, so many of them wouldn't also have died.

Explanation:

It has been estimated that diseases brought by the Europeans to the Americas led to the loss of 90% of Native Americans as they had no prior immunity to counteract the new diseases.

Europeans also died from those same diseases however as shown above with the death of 80% of them in this instance. De Vaca's host was able to stave off revenge killing on the Europeans by showing the Natives that Europeans were equally affected and so did not spread the disease on purpose.