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?). 

Canine distemper virus ( CDV ) invades the nervous system and replicates in neurons and glial cell of the white matter during a period of severe viral induced immunosuppression.
background
The sentence describes the characteristics of canine distemper virus (CDV), which invades the nervous system, and replicates in neurons and glial cells of the white matter during a period of severe viral induced immunosuppression. This is background information that provides an overview and context for the research topic. Therefore, "background" is the correct category for this sentence..