Definition of Bronchiolitis

Bronchiolitis is swelling and mucus buildup in the smallest air passages in the lungs (bronchioles), usually due to a viral infection.

Causes, incidence, and risk factors

Bronchiolitis usually affects children under the age of 2, with a peak age of 3 – 6 months. It is a common, and sometimes severe illness. (RSV) is the most common cause. Other viruses that can cause bronchiolitis include:


Some children have infections with few or minor symptoms.

Signs and tests

Signs include:


Sometimes, no treatment is necessary.

Expectations (prognosis)

Usually, the symptoms get better within a week, and breathing difficulty usually improves by the third day. The mortality rate is less than 1%.


Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc. – 11/2/2009

Normal lungs and alveoli

ADAM Medical Encyclopedia

Article written by

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission ( URAC's accreditation program is an independent audit to verify that A.D.A.M. follows rigorous standards of quality and accountability.