The spanish "secret weapon" in the conquest of the aztecs and inca was

Virgin Soil epidemic is the right answer.

The virgin soil epidemic is an epidemic in which people at risk had no prior contact with the virus that hits them, therefore people become immunologically almost helpless.

After the arrival of Cortes in Mexico, another army came for Cortes inspection for the management. In return to this Cortes fought against the army from Cuba and there an African slave had a disease known as smallpox. And that slave came into contact with others. This disease, therefore, spread more instantly in the soldiers of the  Aztec empire.

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