I keep hearing radio advertisements saying they have a shortage of O neg blood. Now I am not O neg, but it strikes me that the risk of not having enough blood is greater than the possible risk of a rare disease being transmitted? I know in an ideal world, it would be perfect, but even now, there are things that can be transmitted in the blood that cannot be detected (or rather, are not actively screened for) like parasites.
AFAIK, I am not barred from bone marrow donations, or organ donations (including living donations) Is that because the risk-reward levels change?