American Lung Association indicates pneumonia is a lung infection caused by bacteria, fungi and viruses. The condition is not a single disease and has more than 30 different causes. Influenza is the most common cause of viral pneumonia in adults, and respiratory viruses cause one-third of all pneumonia cases in the United States, as of October 2014.
Other respiratory viruses that cause pneumonia include rhinovirus, respiratory syncytial virus, herpes simplex and severe acute respiratory syndrome (SARS). These viruses start in the upper respiratory tract but may cause pneumonia in children as the disease spreads, according to ALA.
The most common cause of bacterial pneumonia in adults is Streptococcus pneumoniae. ALA explains dozens of other bacteria cause pneumonia, such as Staphylococcus aureus, Legionella pneumophila and Chlamydophila pneumoniae. People with weakened immune systems may already have these bacteria, but then the infection spreads to the lungs when the body's natural defenses are diminished.
Very small microorganisms called mycoplasms cause mild forms of pneumonia. Mycoplasms exhibit traits of both viruses and bacteria. Tuberculosis is a dangerous lung disease that can lead to pneumonia. ALA reveals other, less common, causes include inhalation of dust, fungi, food, hazardous chemicals, liquids or gases.
Pneumonia occurs when an infection spreads from the nose, mouth or throat to one or both of the lungs. Air sacs in the lungs that exchange oxygen for carbon dioxide fill with fluid and cannot get enough oxygen to the bloodstream, according to ALA. Pneumonia infections spread quickly and can cause death if left untreated.