I shouldn't give blood if I have mad cow disease

To prompt discussion of prions (and reading of the case study in the text) in my 101 class, I told my students that I lived in Scotland 1994-1995 and ate beef and am indefinitely not allowed to give blood because of this. I asked them to answer 1) why living in Scotland has anything to do with giving blood 2) If not being allowed to give blood is a good policy and 3) why or why not.

As I read the responses, I am learning that: It is "more than likely" that I have mad cow disease*, that I could have contracted "who knows what" while living it Scotland (which is apparently rife with communicable diseases), that the policy is bad because "lots of people eat lots of things in other countries" and that the policy is good: "You should not be allowed to give blood if you have mad cow disease".

Alas, some key points are being missed.
*1 confirmed case [ever] in the US- we need to cover a little more probability