Comment by otherme123

The answer to all of that is mainly hepatitis C, that can have a window detection of 6 months, even more.

And yes, you can be very careful and get a disease. But they are playing statistics here: over 60% of injected drug users have Hep-C, that means a lot of prostitutes. They won't and shouldn't trust anyone who say "hey, I had unsafe sex against all advice, but was very careful with the tattoo in a dark cellar and the heroin party, pinky promise".