Slow viruses and chronic disease of the central nervous system.

Although recognized since the 1930s, slow infections have only recently received considerable attention. There are three types, group A (related to the type C RNA viruses), group B (bizarre agents such as scrapie and kuru) and group C (viruses such as measles which normally produce acute infections but which are behaving here in an unusual fashion). These viruses are only united in that they produce disease with excessively long incubation periods. Many slow infections result in neurological diseases and these will be discussed, together with some possible explanations of their action.