There seems to be a case for presenting a more fully developed version of the ideas put forward in a letter to the British Medical Journal (Webb and Smith, 1964) concerning the pos sible causation of central nervous system (C.N.S.) lesions in virus infections. This somewhat speculative article has there fore been written in the hope of stimulating thought, discussion, and further research by clinicians, virologists, and immuno logists. When large numbers of sera from human populations are tested, antibody to viruses, such as poliomyelitis or Japanese encephalitis, which typically cause C.N.S. disease, is often found in people with no history of C.N.S. illness. Inapparent infec tion rates with such viruses vary from 99.9% for poliomyelitis and Japanese encephalitis to about 90 % for eastern and western encephalitis. Perhaps the major feature of meningoencephalitis in man which therefore requires explanation is why only a very small proportion of such infections actually cause a clinically recognizable C.N.S. disturbance. When there is a C.N.S. involvement the illness is funda mentally biphasic: a mild, often missed, febrile illness is fol lowed by a quiescent period which may vary from 1 to 14 days, and then by the C.N.S. disturbance. The initial phase may even be afebrile. The picture in man is perhaps best seen in European biphasic meningoencephalitis, but Kyasanur Forest disease virus in experimental animals (Webb and Chatterjea, 1962) and humans (Webb and Lakshmana Rao, 1961) and Langat virus in humans (Webb et al., 1966) also cause biphasic illnesses. There is a similar course of events in many other virus infections, including poliomyelitis (Horstmann et al., 1954).
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Webb et al. (1966) studied this question.