Alzheimer's disease (AD) is a condition that causes abnormal changes in the brain mainly affecting memory and other mental abilities. Alzheimer's is a disease, not a normal part of aging. Loss of memory is the usual first symptom. As the disease progresses, the loss of reasoning ability, language, decision-making ability, judgment and other critical skills make navigating day-to-day living impossible without help from others, most often a family member or friend. Sometimes, but not always, difficult changes in personality and behavior occur.