A new appreciation of the microbiome is changing the way we perceive human health and disease. The holobiontic nature of humans is even etched into our DNA in the form of viral symbionts. Empirical evidence for the presence of endogenous retroviruses (ERVs) in the human genome and their activity in homeostatic and pathologic states has accumulated; however, no causal relationship with human disease has been established to date. In this review, we will focus on the role of endogenous retrovirus-K in neurologic disease. Specifically, we will attempt to reconcile the pathologic contribution of ERVK in disparate neurologic diseases by providing evidence as to inter-individual differences in ERVK genotypes, addressing the molecular regulation of ERVK, and provide detailed examples of ERVK-mediated processes in nervous system diseases.