(Pulled From http://www.alzinfo.org/alzheimers-research-causes.asp)
What causes Alzheimer's?
We still don't fully understand what causes Alzheimer's disease, but scientists are zeroing in on the answers. This is one of the most exciting - and most important - areas of research, because understanding the causes should lead to more targeted treatments and ways to prevent the disease.
Scientists generally agree that there is unlikely to be a single clear "cause" of Alzheimer's. It is more likely the result of a combination of inter-related factors, including genetic factors (link), which are passed along family lines of inheritance, and environmental influences, which range from previous head trauma (link) to educational level (link) to one's experiences early in life (link). Each of these "risk factors" is currently the subject of a great deal of research. A growing body of research is also helping to identify various "lifestyle factors," such as dietary habits (link), high blood pressure and high cholesterol (link), which may influence one's risk of Alzheimer's disease.
What is clear is that Alzheimer's develops as a result of a complex cascade of biological processes that take place over many years inside the brain.
Stunning progress has been made recently in unraveling this cascade, and scientists now have a much clearer picture of what happens to the brain when Alzheimer's strikes.
Biological processes underlying the disease: What are the primary biological characteristics of Alzheimer's disease? (Link)