In some patients the specific cause of the disease can be diagnosed, but in others the probable cause cannot be determined, a condition called idiopathic pulmonary fibrosis. There is no known cure for the scars and damage in the lung due to pulmonary fibrosis.^[2]

Pulmonary fibrosis may be a secondary effect of other diseases. Most of these are classified as interstitial lung diseases. Examples include autoimmune disorders, viral infections and bacterial infection like tuberculosis which may cause fibrotic changes in both lung's upper or lower lobes and other microscopic injuries to the lung. However, pulmonary fibrosis can also appear without any known cause. In this case, it is termed "idiopathic".^[4] Most idiopathic cases are diagnosed as idiopathic pulmonary fibrosis. This is a diagnosis of exclusion of a characteristic set of histologic/pathologic features known as usual interstitial pneumonia (UIP). In either case, there is a growing body of evidence which points to a genetic predisposition in a subset of patients. For example, a mutation in surfactant protein C(SP-C) has been found to exist in some families with a history of pulmonary fibrosis.^[5] Autosomal dominant mutations in the TERC or TERT genes, which encode telomerase, have been identified in about 15 percent of pulmonary fibrosis patients.^[6]

Diseases and conditions that may cause pulmonary fibrosis as a secondary effect include:^[2][5]

• Inhalation of environmental and occupational pollutants, such as metals^[7] in asbestosis, silicosis and exposure to certain gases. Coal miners, ship workers and sand blasters among others are at higher risk.^[4]
• Hypersensitivity pneumonitis, most often resulting from inhaling dust contaminated with bacterial, fungal, or animal products.
• Cigarette smoking can increase the risk or make the illness worse.^[4]
• Some typical connective tissue diseases^[4] such as rheumatoid arthritis, SLE and scleroderma
• Other diseases that involve connective tissue, such as sarcoidosis and granulomatosis with polyangiitis.
• Infections
• Certain medications, e.g. amiodarone, bleomycin (pingyangmycin), busulfan, methotrexate,^[4] apomorphine,^[8] and nitrofurantoin^[9]
• Radiation therapy to the chest