I had a test recently (which isn't pleasant). I didn't have blood work done, but then I'm a blood doner, so they test for HIV anyway.
I did it because a girl I was seeing last year, up until July called about a month ago and said she had the dreaded warts. I had no symptoms, but thought it was worth getting tested to make 100% sure.
The point is, that I made certain judgement calls whilst I was seeing her. We didn't have sex for about 3 weeks from dating, and in that time I believed her to be educated, sensible and not very promiscuous. All of those things are still correct but she picked up an infection anyway. We never had unprotected sex, but were talking about her going on the pill after about 3 months together, and I would have been happy with that. I can't really imagine saying to someone at the start of a relationship,
"Do you mind having a full check up first and provide documentation that you're free from STI's?".
The tests came back clear by the way....