The quote “... any Indian who received news of the Spaniards could have also easily received the infection” refers to the fact that when the Spanish explorers advanced into Central America, the diseases they were carrying often spread faster than them, including ahead of them.
The context is the so-called Columbian Exchange (after Christopher Columbus), which is the large exchange of animals, food, culture, people, techniques, and diseases that took place between Native Americans (or Indians) and Europeans after 1492.
The quote focuses on one particular aspect of this exchange: the spread of infections. While the Indians did contaminate the Europeans with some of their diseases, like syphilis, many infections destroyed Native American lives because they had not developed a resistance to them like Europeans did. These included measles and smallpox, which were introduced in America through the Caribbean in the early 16th century.
