According to UpToDate, people with croup are considered contagious for at least three days after the onset of symptoms or until there is no longer a fever present. Croup is readily spread through respiratory secretions from coughing and sneezing, and is considered highly contagious.
Croup is most common in children, states UpToDate. The condition is caused by the same viral infections that cause other illnesses in children. However, most people who have these viral infections do not develop croup. Croup is most identifiable by the barking cough and hoarseness that accompanies the infection. A high fever typically accompanies the other respiratory symptoms.