Pneumonia is a condition in which the lungs fill with mucus. It is characterized by respiratory and systemic symptoms, including fever, congestion, cough, shortness of breath, and increasing aches and pains. Specific clinical signs observed during a physical examination include hypoxemia (a lower-than-normal level of oxygen in the blood), a crackling sound when breathing, the production of greenish sputum (mucus), and the appearance of a 'shadow' in the lung on a chest radiograph.