A study showed that if they wired two mice's circulatory systems together, one with Alzheimer's and one without, the second one would eventually develop everyone's favorite brain plaques.

https://www.newscientist.com/article/2151909-alzheimers-may-...

So while wiring together two bloodstreams is a bit extreme, and likely to result in most things being transmissible, there's nonzero evidence that you can have a bad day from a blood transfusion.

There's also been observations of particularly large amounts of certain human herpes viruses in the brains of Alzheimer's patients.

The diabetes/metabolic theory was one pitched theory, and it isn’t even a terribly mainstream one. There is no consensus that Alzheimer’s is a “metabolic disease”. Even thought ApoE is implicated, that association could be explained by numerous mechanism, even viral.

No expert, but the latest studies have been about the correlation with the herpes virus and how the plaque build-ups that are part of Alzheimer's may be the immune system's attempt to protect the brain.