? Many causes can lead to pneumonia, but its most common cause is a viral or bacterial infection. These viruses or bacteria can come from the environment in which they are found or can be spread by other people. Common routes of transmission include direct person-to-person contact (usually by hand) or inhalation of airborne droplets produced by people when they cough or sneeze. Sometimes, a person with a viral infection (such as the influenza virus) may also develop a secondary bacterial infection (such as Staphylococcus aureus). Rarely, pneumonia can also be caused by parasites, fungi, or yeasts. Aspiration pneumonia is caused by foreign bodies, usually food or vomit that enters the lungs via the larynx, which can irritate airway and lung tissue and increase the chance of bacterial infection of the lungs.