What Is It?
Pneumonia is an infection of the lungs. Most pneumonias 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. However, because these less common infections do not always cause all of the classic pneumonia symptoms, they often are called atypical pneumonias. Atypical pneumonias most commonly occur 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. Also, hospitalized patients are apt to be weakened by other illnesses and 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 overdoses.
Most pneumonias cause fever, cough with sputum (coughed-up mucus), shortness of breath and fatigue. In older patients, fatigue or confusion can be the only or most noticeable symptom. In atypical and viral pneumonias, a dry cough without sputum is more common.