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Last Updated: Mar 1, 2019


Pneumonia is a lung infection that causes inflammation of the air sacs (alveoli). This infection is commonly caused by bacteria, viruses, fungi, or mycoplasma. The severity of illness can range from mild to extremely severe.

Picture of lungs with trachea and diaphraghm

Pneumonia is a leading cause of mortality worldwide. The risk of pneumonia is increased in children younger than 2, adults older than 65, people with chronic illnesses, and those with weak immune systems.

Causes and Risk Factors

Pneumonia is classified by the way patients acquire the infection and the specific organisms involved.

Types of Pneumonia


Tertiary bronchi within lung

The following organisms most commonly cause pneumonia:

Risk Factors

Pneumonia is more common in people with the following risk factors:

Symptoms and Complications

The most common symptoms of pneumonia include:

Immunocompromised patients and individuals with certain chronic diseases have a higher risk of developing complications with pneumonia, including:


In patients with pneumonia, a lung examination with a stethoscope may reveal abnormal lung sounds, such as wheezes and crackles. When the symptoms and physical examination suggest pneumonia, X-rays and blood tests may be used to confirm the diagnosis.

Diagnostic Studies


Patients may be treated on an outpatient basis or hospitalized. After a patient is diagnosed with pneumonia, the clinician’s decision whether to hospitalize the patient depends on the patient’s age, other medical problems, type of pneumonia, severity of illness, and risk of complications.


In cases of HAP and HCAP, the hands of health care personnel often spread the organisms to patients. Hand washing and other good hygiene practices must be performed to prevent pneumonia and other infections. Similarly, medical devices and respiratory equipment should be properly cleaned and disinfected.

The pneumococcal vaccine prevents pneumonia caused by Streptococcus pneumoniae. Vaccinations against influenza (flu) and Haemophilus influenzae type B prevent other forms of pneumonia.

Smokers develop pneumonia more often than nonsmokers. Physicians strongly recommend discontinuation of smoking to lower the risk of pneumonia as well as other related chronic diseases.


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Tina Shahian, PhD

Tina is a writer for Innerbody Research, where she has written a large body of informative guides about health conditions.


A communication specialist in life science and biotech subjects, Tina’s successful career is rooted in her ability to convey complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. Tina Shahian’s Linkedin profile.


In her spare time, Tina enjoys drawing science-related cartoons.