While the brain may shrink to some degree in healthy aging, it does not lose neurons in large numbers. In Alzheimer's disease, however, damage is widespread as many neurons stop functioning, lose connections with other neurons, and die. Alzheimer's disrupts processes vital to neurons and their networks, including communication, metabolism, and repair.
At first, the disease typically destroys neurons and their connections in parts of the brain involved in memory, including the entorhinal cortex and the hippocampus. It later affects areas in the cerebral cortex responsible for language, reasoning, and social behavior. Eventually, many other areas of the brain are damaged, and a person with Alzheimer's becomes helpless and unresponsive to the outside world.