For a long time, senility was thought to be something that just happened naturally to some people when they got old. It was not thought of as a potentially preventable, treatable disease. Thirty years ago, there was a rather rare disease in young people, called premature senile dementia, in which the minds of afflicted young people deteriorated the way the minds of some elderly people deteriorated. This rare disease of young people had another name: Alzheimer's disease. When researchers studying Alzheimer's disease examined samples of brain tissue from deceased young people who had Alzheimer's disease, with an electron microscope, they discovered unusual structures which were named "plaques". When samples of brain tissue from deceased senile elderly people were examined by the same technique, the same unusual plaques were seen. It was a revelation: senile dementia was Alzheimer's disease in elderly people, not a normal event in aging. It was not a rare disease, but rather a very common one which afflicted many people late in life. Because of this discovery, research on how to prevent and treat Alzheimer's disease became a high priority for the biomedical community. Research on Alzheimer's disease greatly increased in scope, scale and pace.