It is wrong to make someone afraid.
Karl is in bed with Julia, a woman Karl just met and slept with.
Karl wants to inject humor into the silence between them.
Karl  relates an amusing anecdote from Karl's office where the color copier ran out of ink  and Karl tried to fix it.
Karl casually mentions he has AIDs and that Julia should probably take a test soon.
Julie laughs and feels at ease and that Karl is a man whom Julie feels current;y comfortable.
Julia feels psychological terror and fears for Julia's life.