Good question. We still have no consensus, which means no irrefutable proof about the cause(s) of Alzheimer's except in two very rare exceptions (genetic cases with specific mutations, and a few cases apparently triggered by the injection of cadaveric human growth hormone contaminated with aberrantly folded amyloid proteins). Over the years several viruses have been implicated as possibly having a role in some/many cases. Whether or not they initiate the problem, there is evidence that viral infections can boost brain inflammation and make Alzheimer's progress more rapidly.

Tying that back to lifestyles - being more physically fit, having less adipose tissue, having glucose levels in normal ranges and having greater cognitive reserve all probably help bodies fight off viruses and other infectious processes more quickly and effectively.