Viral encephalitis : familiar infections and emerging pathogens Richard

THE LANCET • Vol 359 • February 9, 2002 • www.thelancet.com 507 Introduction Encephalitis is an unusual manifestation of human viral infection. Thus, whereas many individuals develop systemic viral infections, only a few develop symptomatic infection of the central nervous system (CNS). Viruses vary widely in their potential to produce significant CNS infection. For some viruses (eg, mumps), CNS infection is a common but a relatively benign part of the syndrome. For others (eg, Japanese encephalitis), neurological disease is the most prominent clinical feature of the systemic infection. A third group of viruses are those which commonly cause infection, but only rarely cause encephalitis (eg, herpes simplex virus [HSV]). Lastly, there are viruses for which human infection inevitably and exclusively results in CNS disease (eg, rabies). In addition to acute pathology, other viruses (eg, measles) can cause syndromes of post-infectious encephalopathy.