My understanding is that there is some evidence that Alzheimer's is transmissible (e.g. higher rate of incidence in caregivers and neurosurgeons), but more study is needed.
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.
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.
