Cough, runny nose and sputum are caused by respiratory infections, which may be viral or bacterial. In case of inflammation of the respiratory tract caused by viral infection, the sputum is thin white sputum; in case of inflammation of the respiratory system caused by bacterial infection, thick yellow sputum is more common. When infection occurs, the respiratory mucosa, including the nasal mucosa, tracheal mucosa and bronchial mucosa, will be congested and edematous, and will secrete a large amount of inflammatory exudate. Neutrophils, eosinophils, phagocytes and basophils in the body will also gather at the site of inflammation and continuously engulf foreign bacteria or viruses. These cells then undergo lysis and necrosis and mix with inflammatory exudates to form sputum. Because of the strong irritation of inflammatory mucous membrane, coughing symptoms also occur, and coughing with runny nose and sputum appears.