Note the thickened alveolar walls filled with eosinophilic material. The white areas in this slide are the alveolar air spaces and the pink areas are the walls of the alveoli with excessive quantities of collagen. Such collagen deposition is the basis of the fibrosis. The causes of pulmonary fibrosis are numerous and include asbestos and other inorganic dusts , antibiotics and some viruses. All of these inciting agents lead to an alveolitis in the short term, with a resultant fibroblast activation and ultimately the deposition of collagen. Such fibrosis is seen clinically as restrictive lung disease.