When European explorers and travelers spread out, they brought with them the germs that they had become essentially immune to, that they had learned how to tolerate over hundreds and hundreds of years, thousands of years, of living with domesticated animals who were the sources of those pathogens.
Cuando los exploradores y viajeros europeos se expandieron, trajeron con ellos los gérmenes contra los que estaban inmunizados, los que habían aprendido a tolerar a lo largo de cientos de años, de miles de años viviendo con animales domésticos que eran la fuente de esos patógenos.