A slow virus is a virus, or a viruslike agent, etiologically associated with a disease having a long incubation period of months to years with a gradual onset frequently terminating in severe illness and/or death. A slow virus disease is a disease that follows a slow, progressive course spanning months to years, frequently involving the central nervous system and ultimately leading to death; examples are visna and maedi of sheep, caused by viruses of the genus Lentivirus (family Retroviridae), and subacute sclerosing panencephalitis, apparently caused by the measles virus. Transmissible spongiform encephalopathies (TSEs), including Creutzfeldt-Jakob disease of humans, scrapie of sheep, and bovine spongiform encephalopathy (BSE) of cattle may also be classified under slow virus disease but are now considered to be prion diseases. Characteristics of Slow virus diseases are: They have long incubation periods ranging from months to years. They have slow but relentless progressive courses terminating fatally. They have a genetic pre-disposition. The immune system does not seem to play a role in its protection.

See also

• Clinical latency
• Virus latency