As before, with ground glass opacities there is a long list of causes. Ask yourself acute vs subacute vs chronic? Overall can split etiologies into water, cells/pus, blood, protein. Specific etiologies include: ACUTE: edema, infections (PJP, viruses, opportunistic infections), hemorrhage, ARDS, HP, aspiration. CHRONIC: edema, recurrent hemorrhage, HP, RBILD, sarcoid, NSIP, DIP, aspiration, organizing pneumonia, PAP.