There used to be a very dangerous disease called smallpox. It made people have a high fever and sores or bumps on their skin. It was really bad because about 30% of the people who got smallpox died. But luckily, today, we don't have smallpox anymore because people all around the world got vaccinated against it.
Smallpox helped the Spanish conquerors win their battles against the Aztec and Inca civilizations. In 1520, an African slave who had smallpox went to Mexico with the Spanish soldiers. The disease spread to the capital city of the Aztec Empire, and in just two years, the Native American population went down from 25 million to 1.6 million.
The Spanish soldiers had been exposed to smallpox for a long time, so they were mostly immune to it. But the Native South American people didn't have immunity, so they got very sick.
