Croup is most commonly caused by viral infections that inflame and swell the upper airway, leading to a distinctive barking cough and sometimes noisy breathing. In rare cases, bacteria can be a cause or a secondary complication, especially if a viral infection is present, but vaccines have greatly reduced bacterial causes like Hib and diphtheria in many regions. Viral culprits most frequently include parainfluenza viruses, but other viruses such as RSV, influenza, rhinovirus, and coronaviruses can also trigger croup symptoms. Bacterial croup is uncommon in children in many countries, and antibiotics are not routinely used unless a bacterial infection is suspected or confirmed.

Key points

• Typical age range: most often affects children 6 months to 3 years, though it can occur in others. Small airways in young children contribute to symptom severity.

• Transmission: viruses spread through respiratory droplets; hand-to-face contact after touching contaminated surfaces can also spread infection.

• Common viruses: parainfluenza (most frequent), RSV, influenza, rhinovirus, adenovirus, human metapneumovirus, and others depicted in reputable sources.

• Bacterial involvement: rare causes include Haemophilus influenzae type b and Corynebacterium diphtheriae, with vaccination and high clinical awareness reducing incidence; bacterial infection can complicate viral croup in some cases.

Clinical implications

• Most cases are mild and managed at home with supportive care and, in some cases, a single dose of a corticosteroid to reduce airway swelling. Antibiotics are not appropriate unless a bacterial infection is suspected. Severe cases may require hospital care, possible nebulized adrenaline, and observation.

• Alarm symptoms warranting urgent care: increasing work of breathing, persistent stridor at rest, blue lips, poor oxygenation, dehydration, or signs that the child cannot drink fluids. Immediate medical attention is advised for severe symptoms.

If you’d like, I can tailor this to your location or age group, or provide guidance on what to expect during a visit and typical home care steps.