Coughing up yellow sputum is most commonly associated clinically with inflammation of the respiratory tract caused by purulent bacterial infections, including acute upper respiratory tract infections, bronchitis, pneumonia and even lung abscesses. However, judging by the symptom of coughing up yellow sputum alone does not indicate the presence of bacterial infection alone, because there is also a case of upper respiratory tract infection, where the most common pathogen is still a virus, and as the patient’s resistance decreases, there may be a secondary bacterial infection, and at this time there is a mixed infection with both bacteria and virus. Therefore, when coughing up yellow sputum, there may be other pathogenic infections, such as viruses, and at this time, blood tests can be performed, and even the culture of pathogens can be taken from the yellow sputum, such as the isolation of viruses, to clarify the cause.