system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 why is the reason why people with Alzheimer’s Disease die?

What fails in the body?
gpt
What fails is the brain. 

The brain controls memory, which is the classic thing you think of with dementia, but it also controls inhibitions, motor control, balance, swallowing, breathing...everything your body does is controlled by some part of the brain. 

Different parts of the brain fail at different times; so often memory or judgement are the first to go, and the more basic things like ability to chew and swallow food don't get destroyed until later stages. 

Dementia (Alzheimer's is one kind, there are several) is organ failure in which the organ failing is the brain. If you are asking specifically how Alzheimer's causes the brain to fail, well, lots of research has gone into that over the years and there is still uncertainty. But if you Google "Alzheimer's brain scans" you'll find you can literally see the brain shrink over time.