No, I don't donate blood. I used to, regularly, but now they won't let me, because they don't understand statistics and probability. I once tested positive on a quick and easy and only 75% accurate test for HIV that they subject all blood to, and negative on a more time consuming and accurate test that all the positives on the first test get, but that one false positive was enough. I'm off the list of eligible donors, despite the fact that it was over 15 years ago and I've been in a monogamous relationship for over twice that long and clearly do not have HIV. Pisses me off. Ignorance always pisses me off.

Lemme just explain that. Suppose you test positive for some condition on a test that's known to be 75% accurate, and you're part of a population for which it's known that the actual incidence of the condition is 5%. What is the probability that you actually have the condition? It's a lot less than 75%, Consider: in a population of 100,000, 5000 people will have it and 95,000 will not have it. The test will correctly identify 3750 of the 5000 who have it and miss 1250 of them, and falsely identify 23,750 of the 95,000 who don't have it as having it. In other words, there will be 27,500 positives, of which over 86% will be false. The probability that you actually have the condition is less than 14%.

So, whenever some medico tells you you have some horrible condition, get another opinion.