Alzheimer’s disease is an irreversible, progressive brain disease that slowly destroys memory and thinking skills, and eventually even the ability to carry out the simplest tasks. In most people with Alzheimer’s, symptoms first appear after age 60. Alzheimer’s disease is the most common cause of dementia among older people. Dementia is the loss of cognitive functioning—thinking, remembering, and reasoning—to such an extent that it interferes with a person’s daily life and activities. Estimates vary, but experts suggest that as many as 5.1 million Americans may have Alzheimer’s.