A therapist can't share any of your information unless they deem you to be a threat to yourself or someone else or you signed a release. If you tell your T in session you were smoking and there is no release and it's not a danger to you, then your T can't tell your spouse. However, if they see you smoking on the street, technically I think they can say something since you were doing something that is not legal in a public space. I can't imagine why a T would do this but I suppose it's possible.
With a diagnosis, again if there's a release they can share, but otherwise no. Doctors in the same practice can share information in a clients' records, but a release is needed for to share this info with doctors outside the practice. Insurance companies are privy to this information as well.