When do you make something your business?

I am in a small and puzzled minority on this question about whether to disclose information about another person’s infidelity.

The majority viewpoint seems to be that even though the wronged party deserves to know the truth, since the asker wasn’t the one who actually did the cheating, she has no right to tell another person’s secret.

I really can’t wrap my head around this, so I’m wondering about other scenarios:

(1) If you knew that someone had engaged in a criminal act that caused harm to another person (e.g., a hit and run accident), would that be your business?

(2) If you knew that someone had engaged in a criminal act that didn’t harm anyone else (e.g., illegal file sharing or underage drinking), would that be your business?

(3) Can you think of any act that is legal, but harms someone else, that you would consider your business? Suppose, for instance, a good friend’s husband is an alcoholic, and you learn somehow that he is drinking again. Would you tell your friend, his wife?

(4) Would it be your business if the information being withheld was positive (e.g., you know someone has a winning lotto ticket, and that person happened to miss the lotto numbers)?

If you just want to give yes/no answers that would be interesting, but I’m especially interested in hearing your reasoning, and whether you think it’s consistent with the cheating question.