To define the microglial cells just as macrophages would be an oversimplification because, in addition to their role in defending against bacterial and viral infections, they play a crucial role in the maturation of neural circuits by their “synaptic pruning” function [83]; they also produce brain-derived neurotrophic factor (BDNF) to survey mature neurons, mediate synapses, and remove myelin debris by phagocytosis [83,84]. This evidence concerns the gene BDNF and viral infectious disease.