Examples include autoimmune disorders, viral infections and bacterial infection like tuberculosis which may cause fibrotic changes in both lungs 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". 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. Autosomal dominant mutations in the TERC or TERT genes, which encode telomerase, have been identified in about 15 percent of pulmonary fibrosis patients.Diseases and conditions that may cause pulmonary fibrosis as a secondary effect include:
Inhalation of environmental and occupational pollutants, such as metals in asbestosis, silicosis and exposure to certain gases. Coal miners, ship workers and sand blasters among others are at higher risk. 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
Some typical connective tissue diseases such as rheumatoid arthritis, ankylosing spondylitis, SLE and scleroderma
Other diseases that involve connective tissue, such as sarcoidosis and granulomatosis with polyangiitis
Infections, including COVID-19
Certain medications, e.g.
amiodarone, bleomycin (pingyangmycin), busulfan, methotrexate, apomorphine, and nitrofurantoin
Radiation therapy to the chest

Pathogenesis
Pulmonary fibrosis involves gradual exchange of normal lung parenchyma with fibrotic tissue. The replacement of normal lung with scar tissue causes irreversible decrease in oxygen diffusion capacity, and the resulting stiffness or decreased compliance makes pulmonary fibrosis a restrictive lung disease. Pulmonary fibrosis is perpetuated by aberrant wound healing, rather than chronic inflammation. It is the main cause of restrictive lung disease that is intrinsic to the lung parenchyma. In contrast, quadriplegia and kyphosis are examples of causes of restrictive lung disease that do not necessarily involve pulmonary fibrosis. Diagnosis
The diagnosis can be confirmed by lung biopsy. A videoscopic assisted thoracoscopic wedge biopsy (VATS) under general anesthesia may be necessary to obtain enough tissue to make an accurate diagnosis. This kind of biopsy involves placement of several tubes through the chest wall, one of which is used to cut off a piece of lung to send for evaluation. The removed tissue is examined histopathologically by microscopy to confirm the presence and pattern of fibrosis as well as presence of other features that may indicate a specific cause e.g. specific types of mineral dust or possible response to therapy e.g. a pattern of so-called non-specific interstitial fibrosis. Misdiagnosis is common because, while overall pulmonary fibrosis is not rare, each individual type of pulmonary fibrosis is uncommon and the evaluation of patients with these diseases is complex and requires a multidisciplinary approach. Terminology has been standardized but difficulties still exist in their application.