Pneumonia is an infection that inflames the air sacs in one or both lungs. The air sacs may swell with fluid or pus (purulent material), which can lead to a cough that produces pus or phlegm, a fever, chills, and breathing difficulties. Pneumonia can be brought on by a number of different species, including bacteria, viruses, and fungi. The severity of pneumonia can range from minor to life-threatening. The most vulnerable groups include newborns and young children, adults over 65, and those with health conditions or weaker immune systems.