What did the Spanish who entered Texas unknowingly bring with them that the Native Americans had no defense over?

Explanation: During the period of Spanish rule (1716-1821), Texas was one of four provinces in New Spain, or colonial Mexico. Spain was unable to populate the area north of the Rio Grande; to maintain their claim on the territory, the Spanish relied on a system of Franciscan missions. Over the years, about two dozen missions were built in Texas, with the goal of transforming the Indians into Spanish subjects by teaching them the Roman Catholic religion and other aspects of Spanish culture. Native Texas Indians such as the Coahuiltecans and the Jumanos had little interest in adopting Spanish culture, and they suffered greatly from the epidemic diseases that the Europeans inadvertently brought into their midst. However, they did look to the Spanish to help provide protection from the Apaches and Comanches, two warlike tribes that had only recently entered Texas. Unfortunately, the Spanish seldom had a strong enough military presence to protect the missions from attack.

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