According to the National Institute on Aging, Alzheimer’s is defined as a brain disorder that is both progressive and irreversible. Alzheimer’s disease slowly destroys both memory and thinking skills, and soon those with Alzheimer’s are unable to carry out the simplest tasks. Below is a video from the NIH which explains Alzheimer’s disease and the changes it causes in the brain.
You can check out the National Institute on Aging’s website here. The Alzheimer’s Association is also a great resource for learning about this disease, and their website can be found here.