Viruses that infect the central nervous system may cause acute, chronic, or latent infections. In some cases, the diseases manifested are attributable to viral damage of neurons or supporting parenchymal tissues; in other cases, to immune attack on virally infected cells. They can be spread by excretion, by respiratory droplets or fomites, or, alternatively, by bites of insects or animals. These viruses range from those such as polio (Chapter 1) or rabies (Chapter 3), whose history in man is as old as the earliest records, to those that emerge from animal reservoirs to human hosts for the first time, such as SARS, Hendra, and Nipah viruses (see Chapter 21).
In this section of Neurotropic virus infections, viruses with an RNA genome are described, starting with the simplest, picornaviruses (Chapter 1), to the most complex, alphaviruses (Chapter 6) and flaviviruses (Chapter 7). RNA viruses require an enzyme not found in host cells: RNA-dependent RNA polymerases to generate both sense (mRNA) and antisense RNA copies. Because of the lack of the host cell proofreading capacity for genome copying, errors are frequently introduced. Some of these errors are neutral, others may be deleterious (and are selected against) or, alternatively, potentially beneficial in evading host immune responses ranging from innate immune recognition to host adaptive immune recognition by Th1 or CD8 cells of epitopes expressed by host MHC or MHC I molecules, respectively, or of antibody recognition of native proteins expressed by virions or infected host cells.