A South African guy passed through our neighbourhood bar for a few weeks. I never really talked to him, he was just another guy passing through my watering hole as I drank with the other regulars.
A nurse I knew was going through a nasty divorce, and asked me for a ride home from the bar one night. I dropped her off, and declined her offer to stay for a while.
A few days later, she called me. Her husband, who I also knew, had changed the locks on the house. She asked me to help her break in, which I absolutely refused to have any part in.
Many months later, it turned out that she had had an affair with the South African guy, and contracted HIV.