Croup usually develops as a result of an infection, often by a virus and sometimes by bacteria. The infection causes the larynx (voice box) to become swollen and the trachea (windpipe) to become blocked.
Parainfluenza virus
The parainfluenza virus is the most common virus that causes croup. Three different strains of the virus are responsible for around 80% of croup cases. These are:
- parainfluenza I
- parainfluenza II
- parainfluenza III
Parainfluenza I is the strain responsible for most cases of croup.
The virus can be transmitted through close contact with people who are infected and through infected objects and surfaces. As with many airborne viruses, the parainfluenza virus is often spread when someone who is infected coughs or sneezes and the infected droplets are breathed in.
Other viruses
A number of other viruses can also cause croup. These include:
- influenza A and B (flu viruses)
- the measles virus, in children who have not been immunised against measles
- the herpes simplex virus (cold sore virus)
- the varicella-zoster virus (chickenpox virus)
- the adenovirus
- the rhinovirus
- the respiratory syncytial virus (RSV), which can cause severe breathing problems and pneumonia in babies
- Mycoplasma pneumoniae, which is a bacterium that can sometimes cause pneumonia
Less common causes
Other, less common causes of croup include:
- an allergic reaction to substances such as pollen or dust mites
- inhalation of irritants, such as chemicals
- acid leaking back out of the stomach and into the throat (acid reflux)
