Alzheimer's disease is an irreversible brain disease that is the fourth leading cause of death among older adults. It destroys memory, thinking skills, and the ability to function. About 4.5 million Americans have Alzheimer's disease, and the number is expected to increase by 70% by 2020 as the population ages. The disease causes changes in brain structure and function that can be seen in brain scans. While there is currently no cure for Alzheimer's, symptoms can be temporarily treated with medication.