Ascaris larvae or eggs can be found in sputum, which is a clinical symptom of pulmonary ascariasis. After human accidentally swallowed infected roundworm eggs, they hatch in the small intestine, and the roundworm larvae break out of the shell and reach the lung through 3 ways: 1, through the small intestine mucosa, microvessels, portal vein, hepatic vein, inferior vena cava, right heart and pulmonary artery to the lung; 2, through the small intestine mucosa lymphatic duct, thoracic duct and left heart to the lung; 3, the roundworm larvae pass through the small intestine wall into the abdominal cavity and then enter the lung through the liver, diaphragm and thorax. Ascaris larvae can damage pulmonary capillaries and alveoli when migrating in the lungs, causing punctate hemorrhagic alveoli, fine bronchitis and eosinophil infiltration with increased mucus secretion or the formation of eosinophilic granuloma, and hemorrhagic pneumonia and pulmonary lobular embolism when a large number of infections occur. The adult worms may occasionally enter the thoracic cavity through the liver to the lung, or penetrate the venous system through the right heart to the lung, or through the pharynx, trachea, or bronchi to the lung, causing lung abscess, pulmonary embolism, or asphyxia.