Does Alzheimer's disease by itself cause death,actually ? directly,indirectly,what is the mechanism ??

Alzheimer's disease is a type of dementia that causes the degradation (wasting away) of the brain overtime causing it to loose function, which eventually causes death.

Alzheimer's itself does not cause death; but it does cause the mind to stop working properly, so it can be listed as a secondary cause of death on one's death certificate (and I highly recommend people do so, simply because it's one way to help garner more funding for research).

With Alzheimer's, the mind slowly degrades over time; and in the later stages of the disease, a person no longer can bathe themselves, walk, even talk; and one of the biggest causes of death in an Alzheimer's patient is aspiration pneumonia. That's where the patient inhales food, rather than swallowing, because their mind has forgotten the swallowing process (in some cases they lose the reflex because of other medical issues) and they breathe the food into their lungs, where it brings on pneumonia. Some have fatal MI's (heart attacks). Some fall, and break bones (hips, concussion, etc.).

I lost my dad to Alzheimer's; his death cert listed MI as the primary, with aspiration pneumo and Alzheimer's being secondaries. My FIL's mother also died from Alzheimer's related issues; she had fallen and broken her hip, and right before surgery was to begin she had a fatal MI (most likely due to a blood clot or to septic shock).