When the Spanish conquistadors, led by Hernán Cortés, arrived in what is now Mexico, they brought with them not only a thirst for gold but also unintended viral stowaways. Among these was smallpox, a disease unknown to the indigenous populations of the Americas. Unlike the Europeans, who had developed some immunity over centuries of exposure, the native people had no defense against the virus. The result was catastrophic. Within just a few years, smallpox decimated the Aztec empire, one of the m