Pneumonia is an infection of the lungs that causes inflammation of the air sacs (alveoli), which may fill with fluid or pus and impair oxygen exchange. It can be caused by bacteria, viruses, fungi, or other microorganisms and ranges in severity from mild to life-threatening. Common symptoms include cough (often producing mucus), fever, chills, chest pain that worsens with breathing or coughing, shortness of breath, and fatigue. Older adults, young children, and individuals with weakened immune systems or chronic illnesses are at higher risk for severe complications. Pneumonia can develop on its own or as a complication of other respiratory infections, and early medical evaluation is important to assess severity and prevent complications.