Measles can persist in the central nervous system and cause subacute sclerosing 
panencephalitis (SSPE), a progressive disease that is almost always fatal. The 
clinical findings of SSPE include behavioral changes, ataxia, seizures, and 
mental-motor deterioration that begins several years after natural infection 
with the virus. Measles antibody is found in the spinal fluid, and its presence 
is particularly useful in establishing the diagnosis. The mechanisms of 
pathogenesis of SSPE apparently involves the selection of clones of virus, which 
do not replicate to become complete virus particles. These "suppressed" viruses 
are able to multiply and spread from cell to cell. The immune system of the 
patient is unable to clear this infection. Several hypotheses are advanced to 
explain these events. Further studies are needed, however, to develop a complete 
understanding of the pathogenesis of these diseases. Fortunately, SSPE is 
disappearing in the United States. This apparently is related to the widespread 
use of measles vaccines and to the resulting low frequency of natural measles 
infections.