How it happens..

Alzheimer's is characterized by a continuing loss of neural tissue. It is a terminal disease that occurs in stages, often over the course of a decade. Over time, neurons in the brain, especially those associated with memory in the hippocampus, die. Typically, the loss of tissue occurs before symptoms surface. Eventually, memory loss is not the only manifestation of the disease. Progressively, patients lose the cognitive ability to process complex thoughts. This commonly leads to apparent changes in behavior and personality. In the final stages, the sections of the brain that affect how senses are perceived and where one's long-term memories are stored are degraded. Finally, the brain tissue that controls vital functions, such as breathing, are attacked and compromised, leading to death.

 At the microscopic level, neuritic plaques, neurofibrillary tangles, and amyloid angiopathy are present. "The plaques are patches or flat areas composed of clusters of degenerating nerve terminals arranged around a central amyloid core. The neurofibrillary tangles, found in the cytoplasm of abnormal neurons, consist of fibrous proteins that are wound around each other in a helical fashion. These tangles are resistant to chemical or enzymatic breakdown and they persist in brain tissue long after the neuron in which they rose has died and disappeared (Porth, 2007, p. 959-960). Basically, these three factors kill neurons and they cannot be killed by the body's defense systems. Thus, over time, they accumulate, destroy brain tissue, and spread to other areas of the brain. The fact that these cannot be stopped by the body and propagate is the reason Alzheimer's is a terminal disease with distinct stages

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Sources:
Porth, C., & Porth, C. (2007). Essentials of Pathophysiology: Concepts of Altered Health States (2nd ed.). Philadelphia: Lippincott Williams & Wilkins