Pneumonia is an infection in one or both lungs. Bacteria, viruses, and fungi cause it. The infection causes inflammation in the air sacs in your lungs, which are called alveoli. The alveoli fill with fluid or pus, making it difficult to breathe. Read on to learn more about pneumonia and how to treat it.Infection that inflames air sacs in one or both lungs, which may fill with fluid.With pneumonia, the air sacs may fill with fluid or pus. The infection can be life-threatening to anyone, but particularly to infants, children and people over 65.Symptoms include a cough with phlegm or pus, fever, chills and difficulty breathing.Antibiotics can treat many forms of pneumonia. Some forms of pneumonia can be prevented by vaccines.