Emergent Viral Infections of the CNS

Biological evolution of the microbiome continually drives the emergence of human viral pathogens, a subset of which attack the nervous system. The sheer number of pathogens that have appeared, along with their abundance in the environment, demand our attention. For the most part, our innate and adaptive immune systems have successfully protected us from infection; however, in the past 5 decades, through pathogen mutation and ecosystem disruption, a dozen viruses emerged to cause significant neurologic disease. Most of these pathogens have come from sylvatic reservoirs having made the energetically difficult, and fortuitously rare, jump into humans. But the human microbiome is also replete with agents already adapted to the host that need only minor mutations to create neurotropic/toxic agents. While each host/virus symbiosis is unique, this review examines virologic and immunologic principles that govern the pathogenesis of different viral CNS infections that were described in the past 50 years (Influenza, West Nile Virus, Zika, Rift Valley Fever Virus, Hendra/Nipah, Enterovirus-A71/-D68, Human parechovirus, HIV, and SARS-CoV). Knowledge of these pathogens provides us the opportunity to respond and mitigate infection while at the same time prepare for inevitable arrival of unknown agents.