Pneumonia is an infection of the lungs. Most cases of pneumonia are caused by bacterial infections, and the most common cause in the United States is the bacteria Streptococcus pneumoniae. Other bacteria such as Mycoplasma and Legionella, as well as certain viruses, also can cause pneumonia, which is often called atypical pneumonia because these less common infections do not always cause all of the classic pneumonia symptoms. Atypical pneumonia most commonly occurs in people younger than 40.
Pneumonia that develops when someone is hospitalized for another illness tends to be more serious, because the organisms found in a hospital often become resistant to many antibiotics, and hospitalized patients weakened by other illnesses are less able to fight off the infection.
A type of pneumonia called aspiration pneumonia develops when chemical irritants and bacteria from the mouth or stomach are inhaled into the lungs. It is more common in people who have had strokes and have difficulty controlling their swallowing reflexes or people who are unconscious as a result of alcohol or other drug overdose.