Hi Betty, I am not a medical professional, but my understanding is that most of the time it is not Alzheimer's directly that kills, but a symptom of the Alzheimers.  For instance, as mobility decreases and AD patents become bed bound, they are prone to pneumonia and bed sores which can eventually kill them.  Also the capacity to swallow can degrade, making eating and drinking difficult.  Its easy to choke on fluids which leads to pneumonia, which can kill.  Also, once the person can no longer eat and drink, if the family does not chose a feeding tube or IV fluids, the person will eventually pass from dehydration and/or lack of food.  (If that is the case, pain meds are administered to keep the person comfortable).  Hope that helps!