I donated my blood once. Two weeks later, I got a letter from Canadian Blood Services saying that they didn't want my blood because it had antibodies that indicated that I once had Hepatitis A, but that I got better and was now immune to the disease. (I did not know that I had ever had Hep A ? I asked my doctor about it later and he said that it had probably been one of the fevers I'd had when I was a small child, but that I had fought the disease off before becoming jaundiced.) I understand that they didn't want the blood contaminated with a horrific liver disease, but seriously, I'm immune to it now. I'm pretty much the ubermensch. They should have been tripping over their own feet in their mad rush to claim my super-blood. Instead, I can only ever donate blood for research.
One of my roommates had it even worse, though ? he was born in Scotland in 1984 before moving to Canada. Apparently, the danger that he ate meat contaminated with mad cow disease during the first two months of his life was too great to even draw blood from him.