AboutKidsHealth indicates that the virus that causes croup is contagious for adults. However, the characteristic barking cough does not typically appear. In adults, the symptoms usually resemble a common cold.
AboutKidsHealth explains that the virus causes the upper airways – the larynx and trachea – to swell. Babies and young children have small airways, so the inflammation makes it hard for them to breathe. Other symptoms include a harsh cough, hoarseness, sore throat, fever and congestion. Breathing difficulties and severe coughing seem to be worse at night. Cool humid air or warm moist air often relieve these symptoms. The virus lasts for about one week.