Viruses are the causes of most types of croup. Occasionally, bacteria or an allergic reaction may also result in similar symptoms as croup. The most common viruses are parainfluenza virus (accounting for most croup cases), adenovirus, respiratory syncytial virus, influenza and measles.

Children who were born prematurely or who have a history of breathing problems or asthma are more likely to develop severe croup and may require hospitalisation.