Life stories

Some writers say that their works are at least semi-autobiographical. The question I am pondering is whether it is ethical to use biographical information in one's writing? Can you include information about people you know/have heard about and record it as fiction - or do you need to categorize it as being factual? And if you do, do the players need to be identified, and openly credited, or is it okay to appropriate pieces of their personal histories as indicative of the human condition and experience? Does it matter if the publishing forum is public eg in a blog - or private correspondence? Or is it the intent that matters? Does it vary depending on the subject matter eg a story they would tell about themselves - or one they would not reveal? Is it an invasion of privacy? Or is there a point when someone else's life events can be recorded as background to your own story because the consequences of their actions have flowed through to you? Isn't it amazing the questions that surface when you're revising your CV? (Just kidding - it's not for my CV - I just happen to be working on that at the same time.)