There are more than 200 causes of ILD, which are divided into two categories, one for known causes; the other for unknown causes. 1, known causes of interstitial lung diseases are (1) occupational, environmental-related ILD: inhalation of inorganic dust: silicosis, asbestosis, explosive dust lung and plating lung; inhalation of organic dust: exogenous allergic alveolitis; animal proteins: feeding pigeons or ornithophiles lung; thermophilic actinomycetes: farmers lung, bagasse lung, mushroom lung chip; gas/smoke: nitrogen oxides, sulfur dioxide, metal oxides, etc. (2) ILD associated with lung infections: hematogenous disseminated tuberculosis, Pneumocystis carinii pneumonia, various types of bacteria and viruses (CMV, SARS,, HIV, etc.). (3) Drug/treatment-related ILD: antineoplastic drugs: bleomycin, mitomycin, methotrexate, etc.; antibiotics and chemical agents: furan, sulfonamide, amiodarone, hypoglycemic agents, contraceptives, non-steroidal anti-inflammatory agents, antitussive drugs (phenytoin sodium, carbamazepine), penicillamine, colchicine, tricyclic antidepressants, methadone, radiation exposure, high concentration of oxygen, accidental administration of paraquat, etc. (4) Others: ARDS recovery, chronic left heart insufficiency, chronic renal insufficiency, cancerous lymphangitis, graft rejection. 2. Interstitial lung diseases of unknown etiology (1) Idiopathic interstitial pneumonia: idiopathic interstitial fibrosis/universal interstitial pneumonia (IPF/UIP), desquamative interstitial pneumonia (DIP), acute interstitial pneumonia (AIP),, nonspecific interstitial pneumonia (NSIP), respiratory fine bronchitis with interstitial lung disease (RBILD), cryptogenic mechanized pneumonia (COP ), and lymphocytic interstitial pneumonia (LIP). (2) Granulomatous, vasculitis-related ILD: nodular disease, Wegener’s granulomatosis, etc. (3)Collagen vascular disease-related ILD:rheumatoid arthritis (RA), progressive systemic sclerosis (PSS), systemic lupus erythematosus (SLE), polymyositis and dermatomyositis (PM/DM), dry syndrome, mixed connective tissue disease (MCTD), etc. (4) Others: ILD associated with liver disease, ILD associated with intestinal disease. 3. The pathogens of infection-related interstitial lung diseases are partly viruses, such as CMV, SARS,, HIV, etc. Whether they are infectious or not is related to the physiological characteristics of the infecting virus.