In different countries, there are various reasons, some better than others. Chronic illnesses like hepatitis, having had gay sex, having lived in Britain in 1980's. These contribute some risk to donating blood.
Also having malaria, paying money or sex for drugs, lived in Africa for 6 months, ever had a blood transfusion, dura matter transplant... the list goes on. In Canada, every time you donate you answer about 13 questions on your own, then a nurse asks you about 10 "high risk" questions. You can tell they could recite them in their sleep, and I'm almost at the point where I could as well.
