Croup causes

Overview
Croup is most often caused by parainfluenza virus, primarily types 1 and 3, but other viral and possiblybacterial infections can also cause it.

Causes
The respiratory distress is caused by the inflammatory response to the infection, rather than by the infection itself. It usually occurs in young children as their airways are smaller and differently shaped than adults', making them more susceptible. There is some element of genetic predisposition as children in some families are more susceptible than others.

An entity known as spasmodic croup also occurs, distinct from the infectious variety, due to laryngeal spasms.

Viral
Viral croup/acute laryngotracheitis is caused by parainfluenza virus, primarily types 1 and 2, in 75% of cases. Other viral etiologies include influenza A and B, measles, adenovirus and respiratory syncytial virus (RSV). Spasmodic croup is caused by the same group of viruses as acute laryngotracheitis, but lacks the usual signs of infection (such as fever, sore throat, and increased white blood cell count). Treatment, and response to treatment, are also similar.

Bacterial
Bacterial croup may be divided into laryngeal diphtheria, bacterial tracheitis, laryngotracheobronchitis, and laryngotracheobronchopneumonitis. Laryngeal diphtheria is due to Corynebacterium diphtheriae while bacterial tracheitis, laryngotracheobronchitis, and laryngotracheobronchopneumonitis are usually due to a primary viral infection with secondary bacterial growth. The most common bacteria implicated are Staphylococcus aureus, Streptococcus pneumoniae, Hemophilus influenzae, and Moraxella catarrhalis.