that show less variability—both among individuals and throughout the life-span.
The prospects for a thorough understanding of Alzheimer's disease are good. After an initial stage during which there appeared to be almost too much information, research from various branches of neuroscience should settle into a coherent picture that will be both satisfying in theory and applicable in clinical practice. Further work in this area will continue to be shaped by advances elsewhere in neuroscience and, at the same time, is well situated to help impel those advances.
Three areas of particular interest are the mechanisms at the cellular level that enable learning and memory (see Chapter 7 ), a clearer understanding of how Alzheimer's disease acts in the brain at each stage, and ways to protect specific regions of the nervous system from damage or perhaps even to reverse damage. Another advance in epidemiology would be to gather study populations whose health factors are thoroughly known and homogeneous, to address satisfactorily the two-sided question of what brings on dementia with increasing age and what permits increasing age without dementia. Many specialists in this area feel that the current position of research on Alzheimer's disease resembles that of work on Parkinson 's disease about 15 years ago, when the cause and some effective therapies were just about to come to light. Now, with the course of this disease broadly understood and the therapies continuing to improve, Parkinson 's is on its way to becoming a manageable disease. The mood is hopeful that Alzheimer's will follow suit.
Not all dementias take a steady toll, like Alzheimer's, largely from among the elderly. One form that is spreading through a mixed-age population, on the basis of different risk factors altogether, is AIDS-related dementia (see Plate 2 ).
Human immunodeficiency virus (HIV) was established as the cause of AIDS in 1983. In 1985 it was recognized as a lentivirus—that is, a virus that primarily infects the macro-phages and leads to multisystem chronic disease, including encephalitis (inflammation of the brain). By 1986, it was clear that infection of the nervous system was to be a major feature