»Catch, Twenty-two!

Stanford University, amongst others, refuse blood donations from people who have spent more than three months in the UK over the past two decades (!!) . This is because of variant Creuzfeld-Jacob ("mad cow") disease, for which there is no blood test.

... but there isn't a blood test because vCJD is transmitted through spinal matter, not the bloodstream (-- insert gag about blood sausage here --).