Bronchiolitis obliterans (BO), also called obliterative bronchiolitis (OB) and constrictive bronchiolitis (CB), is a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles (small airway branches) are compressed and narrowed by fibrosis (scar tissue) and/or inflammation. Bronchiolitis obliterans is also sometimes used to refer to a particularly severe form of pediatric bronchiolitis caused by adenovirus.