Some forms of Alzheimer's kill brain cells via choking blood capillaries so there is no blood circulation and no oxygen and toxin buildup (the blood capillaries may still be present, but no blood circulation so the blood is stale).

Some other forms form dried blood plaques to prevent neurotransmitters from reaching their intended neurons so signals terminate prematurely (dried blood plaques probably can be considered as a form of toxin buildup so blood capillaries getting choked is probably a reason).

So people who do not exercise enough probably will get Alzheimer (though old people exercising will put them at risk of stroke so old people getting Alzheimer is expected, but exercises like taichi and yoga probably can be done by old people to keep off Alzheimer's).