A new study from Yale has found a link between Alzheimer's and prion 
proteins. NPR is reporting this today as a similarity between "Mad Cow 
Disease" (BSE) and Alzheimer's, although near the end of the piece they say 
there isn't necessarily a link (the story isn't online yet, so I can't 
double-check their wording).

Prion proteins occur in everyone's cells, and normally don't cause a 
problem. In the case of BSE, something causes the prion protein to form in 
a different shape, and the result is it causes the disease.

It is possible that prion proteins are susceptible to being influenced in a 
number of ways, with varying results. I suspect (and certainly hope) there 
is not a causative link between BSE and Alzheimer's.

This is exciting news from the Alzheimer's front, because it provides a new 
direction for research and implicates a well-studied protein. However, 
since prion proteins are also involved in BSE we will be seeing more BSE 
confusion in the news.

Here's the Yale research...
http://health.usnews.com/articles/health/healthday/2009/02/25/scientists-spot-new-clue-to-alzheimers.html