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This is basic probability (bayes formula). But, you need to have the individual probabilities first.

BTW, I wouldn't be too sure about the vast majority feeling like 1 in a million is a small...

Sabio,

My bad, missing that.

As for my opinion, I think I would be too afraid of the unprotected sex. There is that incubation period during which these tests won't come up positive, but during...

I'd like to point out that you are only considering STD's that the girl may have. What about you? How often do you test yourself? Are you possibly the cause of the girl's becoming infected? I think...