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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

  • Community-acquired pneumonia (CAP) develops outside of a hospital or health care facility.
  • Hospital-acquired pneumonia (HAP), or nosocomial pneumonia, develops within 48 hours after a hospitalization. The use of a mechanical ventilator is the main risk factor for the development of HAP. The germs associated with HAP are often resistant to antibiotics, and the mortality rate for HAP is high.
  • Health care-associated pneumonia (HCAP) originates at any type of health care facility, including a nursing home or outpatient clinic. The germs associated with HCAP also tend to be resistant to antibiotics and associated with a high mortality rate.
  • Aspiration pneumonia occurs when patients accidentally inhale their own vomit, saliva, food, or liquids. This condition usually occurs when something impairs the normal gag reflex. For example, people with an altered level of consciousness, brain injury, intoxication, or swallowing difficulty have a high risk of aspiration.


Tertiary bronchi within lung

The following organisms most commonly cause pneumonia:

  • Bacteria. In adults, bacteria (including Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae) are the most common cause of pneumonia.
  • Viruses. Viruses are the most common cause of pneumonia in children younger that 5. Such viruses include respiratory syncytial virus (RSV), parainfluenza virus, and influenza A virus. In adults, the most common causes of viral pneumonia include Influenza A, Influenza B, and RSV. Usually, viral pneumonia causes less serious illness than bacterial pneumonia. However, pneumonia related to influenza is often serious or life threatening, especially in patients with lung disease or heart conditions.
  • Mycoplasma. Certain organisms called mycoplasma are similar to bacteria. Mycoplasma pneumoniae causes a mild case of pneumonia, called “walking pneumonia,” and usually occurs in older children and adults younger than 40.
  • Fungi. Bird droppings and soil contain fungi that can cause pneumonia when the organisms are inhaled. People with weak immune systems are more susceptible to this form of pneumonia.

Risk Factors

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

  • Age younger than 2 years or older than 65
  • Asthma
  • Chronic obstructive pulmonary disease
  • Heart disease
  • Diabetes
  • Mechanical ventilation
  • Smoking
  • Immunocompromised conditions (such as HIV, cancer, organ transplantation, chemotherapy usage, steroid usage).

Symptoms and Complications

The most common symptoms of pneumonia include:

  • Cough
  • Thick mucus (phlegm)
  • Shortness of breath
  • Fever
  • Chest pain
  • Muscle aches
  • Rapid heart rate
  • Confusion.

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

  • Difficulty breathing that requires mechanical ventilation (respiratory failure)
  • Spread of bacteria into the blood (bacteremia)
  • Formation of a pocket of pus in the lung (lung abscess)
  • Accumulation of infected fluid in the lining around the lung (pleural effusion).


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

  • Chest X-ray provides an image of the infected lung area (infiltrate).
  • Blood cultures detect the presence of bacteria in the blood.
  • Sputum is examined with a microscope to determine the type of infection.
  • Pulse oximetry measures the blood oxygen saturation level.
  • Pleural fluid samples are examined for evidence of infection.
  • Bronchoscopy is performed by placing a thin flexible camera through the throat into the lungs to examine the airways and obtain samples of respiratory secretions.
  • Chest computerized tomography (CT) provides detailed images of the tissues.


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.

  • Bacterial pneumonia is treated with antibiotics, which are started as soon as possible. Most patients improve after 1 to 3 days of treatment.
  • Viral pneumonia cannot be treated with antibiotics; instead, antiviral medication is required. Symptoms of viral pneumonia usually improve in 1 to 3 weeks.
  • Mycoplasma pneumonia is treated with antibiotics. Most patients have a mild illness and do not require hospitalization.
  • Fungal pneumonia is treated with antifungal medications. Immunocompromised patients are more likely to develop this infection, and the risk of complications is high.


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.


  • Donowitz GR. Chapter 64. Acute Pneumonia. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 7e. Philadelphia, PA: Elsevier Saunders; 2010.
  • Pneumonia. Centers for Disease Control and Prevention website. Accessed May 21, 2014.
  • What is Pneumonia? National Institutes of Health; National Heart, Lung, and Blood Institute website. Accessed May 21, 2014.
  • Pneumonia. Mayo Clinic website. Accessed May 21, 2014.

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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.