To summarize what I have said so far, Alzheimer's appears to be a consequence of an inability of neurons to function properly, due to a deficiency in fats and cholesterol. A compounding problem is that the fats over time will become rancid if they cannot be adequately replenished. Rancid fats are vulnerable to attack by microorganisms such as bacteria and viruses. Amyloid-beta is part of the solution because it allows the astrocytes to be much more effective in utilizing glucose anaerobically, which protects the internally synthesized fats and cholesterol from toxic oxygen exposure, while at the same time providing the energy needed both by the astrocyte for the synthesis process and by neighboring neurons to fuel their signal firings.
Besides the astrocytes, the microglia in the brain are also implicated in Alzheimer's. Microglia promote neuron growth when all is well, but trigger neuron programmed cell death in the presence of toxic substances secreted by bacteria such as polysaccharides . Microglia will defensively secrete cytokines (communication signals that promote an immune response) when exposed to infective agents, and these in turn will lead to inflammation, another well-known feature associated with Alzheimer's . The microglia are able to control whether neurons should live or die, and they surely base this decision on factors related to how well the neuron functions and whether it is infected. Once enough neurons have been programmed for cell death, the disease will manifest itself as cognitive decline.