Sputum consists of a mixture of necrotic material, bacteria, viruses, and fluid secreted by mucus cells in the normal airways. The presence of sputum indicates the presence of inflammation or congestion or edema in the airways, while the change in the nature, amount and smell of sputum can be helpful in diagnosing diseases. White sputum indicates the presence of chronic bronchitis and cold, and if white sputum changes to yellow sputum or turns purulent when coughing, it indicates combined bacterial infection. A large amount of yellow purulent sputum is common in bronchial dilatation or lung abscess; red sputum, especially jelly-like sputum, often suggests pneumonia caused by Klebsiella; pink foamy sputum is common in pulmonary edema; rust-colored sputum is a manifestation of Streptococcus pneumoniae; green sputum generally refers to yellow-green, and the simple appearance of green sputum is less common, usually caused by Pseudomonas aeruginosa infection.