With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: As the 'rules' of colonization became established into legal doctrine agreed upon by between European colonial powers, methods of laying claims to indigenous lands continued to expand rapidly. As encounters between European colonizers and indigenous populations in the rest of the world accelerated, so did the introduction of infectious diseases, which sometimes caused local epidemics of extraordinary virulence. For example, smallpox, measles, malaria, yellow fever, and other diseases were only seen in post-Columbian Americas and Oceania.
text_B: If someone had smallpox in pre-Columbian times, is it likely they were from Europe?
YES.