Croup? Is it a disease or symptom?

The term croup actually denotes a group of diseases collectively referred to as “croup syndrome,” all of which are acute and predominantly viral respiratory illnesses characterized by marked swelling of the subglottic region of the larynx. Croup primarily affects children <6 years old. For a detailed discussion of this entity, the reader is referred to a text of pediatric medicine.

Source: Harrison_s_Principles_of_Internal_Medicine_16th_Edition