Alzheimer's disease is not the only condition associated with neurofibrillary tangles and senile plaques. To some extent, their development is a part of normal, healthy aging. Many people over 70 have some evidence of tangles and plaques, yet they show none of the cognitive decline of Alzheimer's. Currently, no one knows why.

In addition, the brain changes of Alzheimer's also develop in people with Down syndrome. On autopsy, the brains of Down and Alzheimer's sufferers are often indistinguishable. The major difference is that in Down syndrome, the brain changes develop much earlier in life.

Down results from a chromosome abnormality before birth. Instead of having two of each chromosome, the double-helix repositories of genetic information coded in DNA, their cells contain three copies of chromosome 21. Affected individuals develop a characteristic physical appearance, and a particular form of mental retardation. If they live long enough -- into their 40's or 50's -- people with Down syndrome almost always develop Alzheimer's disease.