Chronic pneumonitis of infancy

**Definition:** Chronic pneumonitis of infancy is a rare paediatric form of interstitial lung disease (ILD) sharing clinical and radiologic features with other forms of ILD (cough, tachypnoea, and infiltrative opacities on chest imaging) and harbouring specific histological abnormalities including diffuse thickening of alveolar septa, hyperplasia of type 2 alveolar epithelial cells (AEC), and presence of primitive mesenchymal cells within the alveolar septa.