Pnömoni is an infection that inflames the air sacs in one or both lungs. These air sacs, called alveoli, can fill with fluid or pus, making it difficult to breathe. Common symptoms include cough, fever, chills, and difficulty breathing. The infection can be caused by various organisms, including bacteria, viruses, and fungi. Bacterial pneumonia is often treated with antibiotics, while viral pneumonia may require antiviral medications. Vaccines are available to help prevent certain types of pneumonia, such as those caused by Streptococcus pneumoniae.