Sharing needles and having casual unprotected sex are dangerous and foolish
Casual unprotected sex wasn't the only sexual risk-- the others were rape and having unprotected sex with an untrustworthy long term partner.
Also blood transfusions. It took the red cross a disgracefully long amount of time to start screening donors. Some people objecting to the screening compared it to the WWII-era separation of blood into black plasma and white plasma.
A post from Gregory Cochran's and Henry Harpending's excellent blog West Hunter.
The commenter Ron Pavellas adds:
The Wasserman Test.