Alzheimer's disease is a brain disorder that slowly destroys memory and thinking skills, and is the most common cause of dementia among older adults with symptoms first appearing in their mid-60s. It is caused by changes in tau protein fibers that twist abnormally and other factors like age, genetics, gender, and cognitive defects. Symptoms include loss of memory, repeating sentences, misplacing items, forgetting names, losing track of time, and changes in personality. While there is no cure, risk can be reduced by not smoking, managing vascular risk factors, eating a healthy diet, and taking care of mental health.