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What is Alzheimer disease?

Alzheimer disease (AD) refers to slowly progressive loss of nerve cells (neurons) in the brain. Symptoms of the illness usually unfold slowly over a period of years. Neurons involved in memory and thinking are affected first, while movement and sensation are only affected in the very late stages. Several different forms of Alzheimer disease have been discovered – a few rare forms are strongly inherited and become symptomatic in mid-life, while the vast majority result from complex interactions between genes and the environment, and do not become symptomatic until late-life.