Sputum is a thick fluid produced in the lungs and airways, often expelled through coughing. It contains mucus, cells, and other substances, and is typically a sign of respiratory conditions. Doctors analyze sputum to diagnose infections, such as pneumonia or tuberculosis, and to assess lung health. The color and consistency of sputum can vary, providing clues about underlying health issues. For example, yellow or green sputum may indicate an infection, while clear sputum is often associated with allergies or irritants. Collecting a sputum sample can help healthcare providers determine the best treatment options.