Alzheimer's disease invades the brain from the inside. Unlike an abcess, a metas- tasis or an infarct, the disease follows specific tracks and avoids certain cortical areas while flourishing in others. Any observer is struck by the exquisite selectiv- ity of the lesions and could, indeed, conclude that Alzheimer's disease knows neuroanatomy. However, should the term "disease" be used to define this disor- der? Several genes, located on at least three different chromosomes, have been implicated in the disease. The ApoE4 genotype has been shown to be an impor- tant risk factor, but dementia...