Pneumonia is an infection process that inflames the air sacs in one or both lungs. These air sacs may fill with fluid or pus thereby leading to a cough with the expulsion phlegm or pus, fever, chills, and difficulty breathing. Pneumonia can be caused by many different organisms, including bacteria, viruses and fungi and can be a side-effect or complication of surgery. It is treated with antibiotics but can cause permanent damage to the lungs.