I am not allowed to give blood, because i once got a blood transfusion in the eighties and i have a tiny, tiny risk of infecting people with mad cow disease.
I only found out when i decided to give blood some years back and found out i am not allowed to.
I can't donate blood because my grandma died from prions. I probably have a much, much higher chance of spreading it than you, and mine is still probably extremely low.
Mad cow disease can (sometimes/possibly) take decades to have an effect. If you are some infected meat and have the wrong genetics you could wind up with a sponge for a brain 30 years later.