Alzheimer's disease is a debilitating brain disease that impairs memory, thinking and behavior. It's the most common form of dementia, a group of brain disorders that cause a progressive loss of brain function in a formerly unimpaired person, and unfortunately, gets worse over time. What's happening behind the scenes inside the brain of a person with Alzheimer's is rather straightforward. Nerve cells (neurons) in the brain die, leading to a decrease in the chemicals (neurotransmitters) that help send the brain's signals. This process makes different areas of the brain lose function, resulting in dementia.