Pneumonia is an infection that inflames the air sacs in one or both lungs, which can fill with fluid or pus, leading to symptoms such as cough, fever, chills, and difficulty breathing. It can be caused by a variety of pathogens, including bacteria, viruses, and fungi. Treatment typically involves antibiotics for bacterial pneumonia, while viral pneumonia may require antiviral medications or supportive care. Pneumonia can range from mild to severe and is particularly dangerous for young children, the elderly, and those with weakened immune systems.