View full lesson: http://ed.ted.com/lessons/what-is-alzheimer-s-disease-ivan-seah-yu-jun Alzheimer's disease is the most common cause of dementia, affecting over 40 million people worldwide. And though it was discovered over a century ago, scientists are still grappling for a cure. Ivan Seah Yu Jun describes how Alzheimer's affects the brain, shedding light on the different stages of this complicated, destructive disease. Lesson by Ivan Seah Yu Jun, animation by STK Films.