OK, yes, I know that. But these are totally arbitrary numbers (3 months, 5 years). Data (via Wikipedia) suggests that 1 in 2,000 people in the UK may be carriers of something that might in very rare cases cause a problem.

I'm O- which means my blood could be used for anyone.

If I were in a situation where I needed blood and I was told that there's a 1 in 2000 chance that I might become a carrier of something which in turn has a 1 in 200 chance of giving me vCJD (rough numbers, based on 170 cases ever in the UK), or I die, then surely this isn't an issue? It's a one in 400,000 risk! I take that much risk crossing the road every year.