Is croup contagious to adults? My niece has it she lives upstairs

Yes. Croup is usually triggered by one of several upper respiratory viruses that are all contageous.However, the croup like illness is rather age specific, occuring most fro age 6 m to 6 yr. Older kids or adults tend to have cold like symptoms or laryngitis if they get the germ. If a person has been infected by that virus strain before they would be immune.