The etiology of multiple sclerosis (MS) is unknown, and several hypotheses have been advanced over the past century to explain it. Despite much effort, no single cause has been established. One of the most appealing hypotheses is that of infection due to a neurotropic infectious agent, possibly a virus. There are several lines of data to support this hypothesis. First, there are clear examples of inflammatory demyelinating disease caused directly or indirectly by viral infections in both humans and animals. Second, there is a strong environmental component to multiple sclerosis. Finally, there is aberrant immune reactivity against various viruses. Recent candidates have included measles and the related canine distemper virus, human herpesvirus 6, human endogenous retroviruses, and Epstein-Barr virus. The evidence is most extensive for the latter and will be discussed in some detail.
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