A variety of viruses infect the human nervous system, often with severe consequences. While vaccines have largely defeated the paralysis caused by polio, other viruses such as rabies, West Nile virus, and herpes simplex virus (HSV) continue to cause neurological infections that require clinical intervention. More than 70% of adults in the United States carry HSV, whether they know it or not. HSV causes recurrent genital and oral lesions (e.g. cold sores), and in rare cases can progress to cause potentially fatal brain infections.