I. Disease pathology The lesions mainly occur in the subsonic cavity, and the inflammation may accumulate in the trachea downward. The mucosa of the subsonic cavity is edematous, and in severe cases, submucosal cellulitis, purulent or necrotic changes may occur. The mucosa may be extensively defective due to ulceration, and pseudomembrane formation on the surface is rare. Symptoms and signs Laryngeal obstruction degree: First degree: the child is quiet as normal, only after the activity of inspiratory laryngeal tones and inspiratory dyspnea, clear breath sounds on auscultation, normal heart rate. Second degree: laryngeal tinnitus and inspiratory dyspnea appear when the child is quiet, laryngeal conduction sounds or tubular breath sounds can be heard on auscultation, and the heart rate is faster, up to 120-140 beats/min. Third degree: In addition to the second degree symptoms, paroxysmal irritability, cyanosis of lips and nails, blue or pale around the mouth, diminished or inaudible respiratory sounds in both lungs on auscultation, blunted heart sounds, heart rate of 140 – 160 beats per minute. Fourth degree: from restlessness to semi-consciousness or coma, with temporary quietness, grayish face, almost disappearance of two respiratory sounds on auscultation, only tracheal conduction sounds, weak heart sounds, and irregular or fast or slow heart rhythm. The differential diagnosis should be distinguished from the following diseases: (1) Tracheobronchial foreign body: sudden onset, with a history of foreign body inhalation. The child has symptoms such as severe cough and dyspnea. Chest auscultation and X-ray machine bronchoscopy can distinguish the two diseases. (2)Laryngeal diphtheria: laryngeal diphtheria has a slow onset, low fever, obvious symptoms of systemic toxicity, pale face, depression, thin and rapid pulse, often with grayish-white pseudomembrane in the pharynx, and the bacillus diphtheriae can be found by taking secretions for examination. (3) laryngospasm: common in younger infants, rapid onset, inspiratory laryngeal wheezing, vocal tone is sharp and thin, the attack is short, the symptoms can disappear suddenly, no hoarseness. Prevention of disease 1, usually strengthen outdoor activities, see more sunlight, enhance physical fitness, improve the ability to resist disease. 2, pay attention to climate change, timely increase and decrease clothing, avoid cold and heat. 3.During the epidemic of cold and flu, go out as little as possible to prevent infection. 4, life should be regular, diet, regular living, sleep at night and get up early to avoid catching cold. When sleeping, avoid blowing convective wind.