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General Considerations

Encephalitis is defined by the presence inflammation of the brain in association with clinical evidence of neurologic dysfunction. Of the pathogens reported to cause encephalitis, the majority are viruses. More than 100 viruses, in various families, may cause encephalitis. Viruses can cause encephalitis through acute primary infection or through a parainfectious or postinfectious immune-mediated response. Based on retrospective International Classification of Disease data, an estimated 263,352 encephalitis-associated hospitalizations occurred in the United States from 1998 to 2010; this corresponds to an average of 20,258 encephalitis-associated hospitalizations per year. Herpes simplex virus type 1 (HSV-1), arthropod-borne viruses (arboviruses), West Nile virus, and enteroviruses are the most common causes in adults. Table 27–1 outlines the epidemiology and clinical features of selected viruses that cause encephalitis.

Table 27–1.Viral encephalitis: epidemiology of selected causes.

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