Congenital AIDS results from active maternal infection even though the infant's mother may be asymptomatic when the first sign of infection presents in her child. In most instances the initial symptoms are not referable to the nervous system, however this may be misleading due to the age of the patient. By eighteen months of age over 90% have evidence of static or progressive encephalopathy. This is almost always due to HIV infection of the central nervous system (CNS) since secondary CNS infections are uncommon in children with AIDS.