TASK DEFINITION: In this task, you are given a sentence from the research paper and your task is to classify the given sentence into the following categories: Background (Why is this problem important? What relevant works have been created before? What is still missing in the previous works? What are the high-level research questions? How might this help other research or researchers?), Purpose (What specific things do the researchers want to do? What specific knowledge do the researchers want to gain? What specific hypothesis do the researchers want to test?), Method (How did the researchers do the work or find what they sought? What are the procedures and steps of the research?), or Finding (What did the researchers find out? Did the proposed methods work? Did the thing behave as the researchers expected?). 
PROBLEM: populations encountered distinct pathogens , and natural selection increased the prevalence of alleles that are advantageous in the new ecosystems in both host and pathogens.

SOLUTION: background

PROBLEM: and 78 were enrolled in this study from August 2003 to March 2018.

SOLUTION: method

PROBLEM: and five viruses found in the subspecies Nyctereutes procyonoides ussuriensis in its original and newly occupied habitat or in Nyctereutes procyonoides koreensis in its original habitat , respectively.

SOLUTION:
background