Pneumonia is an infection that inflames the air sacs in one or both lungs. The air sacs may fill with fluid or pus, causing cough with phlegm, fever, chills, and difficulty breathing. Various organisms, including bacteria, viruses, and fungi, can cause pneumonia. Treatment depends on the cause and severity but may include antibiotics, antiviral medications, or antifungal medications. Prevention includes vaccination, good hygiene, and maintaining a healthy immune system.