_  Studies on Brain imaging suggest that Alzheimer’s and other forms of most prevalent dementia’s may spread within nerve networks in the brain by moving directly between connected neurons, in a way  by propagating in all directions. Studies concluded that a nerve region's connectedness to a disease hot spot trumps overall connectedness, spatial proximity and loss of growth-factor support in predicting its vulnerability to the spread of Alzheimer’s.

http://memory.ucsf.edu/blog/alzheimer%E2%80%99s-disease-spreads-through-linked-nerve-cells-brain-imaging-studies-suggest-522/