Viruses are now known to consist of only two major components; the nucleic acid which is essential for infectivity and an external covering to protect this. Visualization of this external shell by electron microscopy has revealed that viruses display distinctive arrangements of sub-units by which they can be both recognized and characterized. This has made it possible to classify viruses according to their structural and chemical features and such a classification has superseded earlier and essentially biological classifications without contradicting them.
The action of a virus upon a cell may take one of three forms. The virus in the process of replication may destroy the cell which it has infected (lytic action). A second possibility is that it may transform the cell to malignancy (oncogenic action). A third possibility is that the virus may remain latent within the cell for long periods with no obvious manifestations of its presence (symbiotic relationship). It is this last type of interaction between a virus and the host cell which appears to be of particular interest in the context of the nervous system.
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