Almost it's prion-like but it's not exclusively infectious or self-replicating. Tau and beta are more like a "nucleation" sites for junk the glymphatic system can't clear. More like tartar buildup on teeth plus a biofilm.

It would be determine with certainty if Alzheimer's were or were not contagious short of cannibalism. It could be unwittingly being transmitted to those who are susceptible.

Also, we know there are myriad of specific and some unknown health and lifestyle factors that holistically in total tip the scales towards or away from Alzheimer's.