Yellow sputum is often seen in respiratory tract bacterial infections, such as acute bronchitis, pneumonia, pharyngitis, sinusitis and so on. 1. Acute bronchitis: it is an infectious disease mainly caused by bacterial infection of the bronchial mucosa, which mostly occurs in the cold season or when there is a sudden change in the climate. At the beginning, it is a dry cough or a small amount of mucous sputum, and then a large amount of yellow sputum, fever, sore throat and other symptoms may appear. 2. Pneumonia: when bacteria invade the lungs, inflammation of the lungs can occur, the symptoms are mainly coughing, coughing up sputum, fever, wheezing, and most of the sputum coughed up is yellow sputum. 3. Pharyngitis: due to bacterial stimulation of the mucous membrane of the throat, thus causing inflammation, the patient may appear pharyngeal swelling, pain, coughing yellow sputum and other symptoms. 4. Sinusitis: often secondary to viral upper respiratory tract infections, the patient will appear sinus redness, pain, yellow pus nasal discharge symptoms, when the nasal discharge backflow to the throat, easy to cause yellow sputum. If the yellow phlegm is more, you should go to the hospital in time to identify the cause of targeted treatment.