[Answer] 4. This disease devastated the Americas when European settlers introduced it in the fifteenth century.

Step 1 : Introduction to the question "4. This disease devastated the Americas when European settlers introduced it in the fifteenth century."

...1. Herpes Zoster 2. River Blindness 3. Rabies 4. Smallpox Of all diseases brought into the New World, this was the most virulent and caused the deaths of millions of Native Americans. The Aztec and Incan civilizations were decimated, which was a major factor in their being conquered by the Spanish. In the 18th century, it is estimated that Smallpox killed about 60,000,000 people in Europe alone. There is the opposite theory that the pox was brought from the New World to the Old. Either or both could be true. In light of the possibility of germ warfare and the creation of mega-viruses, the Center for Disease Control in Atlanta, GA., keeps the Smallpox virus safely in level four containment, although the World Health Organization has certified it 'eradicated' as of 1979. Source: WHO, Encyclopediias and word of mouth from people who had Smallpox.

Step 2 : Answer to the question "4. This disease devastated the Americas when European settlers introduced it in the fifteenth century."

Smallpox:

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