Pneumonia is an infection caused by bacteria, viruses, or bacteria that could either affect one or both lungs. Viral pneumonia and bacterial pneumonia may spread to others by sneezing or coughing airborne droplets. This form of pneumonia is also possible by getting direct encounter with surfaces or items infected with bacteria or viruses that cause pneumonia. This may spread from one person to another. Pneumonia causes inflammation of air sacs that are called alveoli in your lungs. The alveoli are filled with fluid or pus, which makes breathing difficult for a person.