I gave blood regularly for several years after I was old enough to do so, but then the "Mad Cow Disease" scare erupted in England, where I was born, and the Red Cross here in America stopped accepting donations from anyone who might have eaten beef from England between the years when they think the threat was present. The prions (misfolded proteins active in the brain) that cause "Mad Cow" can, in humans, cause variant Creutzfeldt-Jakob disease. Since it's difficult to screen for, the Red Cross simply stopped accepting donations from those who might have been infected. Fortunately the disease is extremely rare in humans, but it has a 20-year incubation period. It's now been more than 20 years since I lived in England, so it should be safe for me to donate once more. I guess I should look into it.