Pneumonia (nu-MO-nyahnu-MO) is an inflammation of the lungs. It is a common illness usually caused by infection with a bacterium, virus, or fungus. It is often mild, especially in young people. Pneumonia may also cause serious illness, especially in people who are old or already have health problems, and it remains a major cause of death.

Front view chest x-ray showing the lower lobe of the lung affected by pneumonia (left).

Front view chest x-ray showing the lower lobe of the lung affected by pneumonia (left).
PhotoStock-Israel/Science Source.

What Is Pneumonia?

When a person breathes, air enters the lungs and travels through millions of tiny sacs. In these sacs, known as alveoli (al-VEE-oh-lye), oxygen * is transferred to the blood, which carries it to all parts of the body.

When someone has pneumonia, the lung tissue becomes inflamed and the alveoli fill with mucus * and other debris, making it difficult for oxygen to be transferred. Breathing is difficult and the lungs have to work harder to take in oxygen and to exhale carbon dioxide, a dangerous waste product in the blood. Eventually, the cells of the body may not get enough oxygen, and carbon dioxide may build up in the body.

Normally, the body has ways to fight off lung infection. These include the following:

These defenses usually prevent pneumonia, but sometimes a person's defenses may be weakened by illness, age, or other factors. The defenses also may be overwhelmed by a particularly heavy or virulent * dose of germs.

What Are Some Different Types of Pneumonia?

More than 75 kinds of germs can cause pneumonia. Bacteria and viruses are the most common causes. Fungi and parasites are less common causes. Chemicals, drugs, and radiation can also cause lung inflammation. Aspiration (as-puh-RAY-shun) pneumonia occurs when someone accidentally inhales food or vomited material into the lungs.

Bacterial pneumonia

Other bacterial causes include the following:

Viral pneumonia

Viral pneumonia, usually seen in children, makes up nearly half of all pneumonia cases. This type of pneumonia is usually not severe or long lasting. Causes of viral pneumonia include the following:

Fungal pneumonia * . Other fungal infections that can involve the lungs are histoplasmosis (his-toh-plaz-MOH-sis), blastomycosis (blas-toh-myeKOH-sis), coccidioidomycosis (kok-sih-dee-oyd-oh-mye-KOH-sis), and aspergillosis (as-per-jih-LOH-sis).


Sir William Osler (1849–1919), often called the father of modern medicine, spent much of his life studying pulmonary diseases (diseases of the lungs) and is known for calling pneumonia the “old man's friend.” He gave pneumonia this grim nickname because it swiftly ended the lives of many old men and women who had been suffering with other untreatable diseases. Ironically, Osler himself died of pneumonia after contracting influenza.

The United States and the World

Pneumonia was a leading cause of death in the United States in the early decades of the 20th century. By 2015, antibiotics and good medical care usually cured pneumonia, even in the sick and elderly. Yet pneumonia occurred in so many people that, combined with influenza, it still ranked as the seventh most common cause of death in the United States. An estimated 2 million Americans got pneumonia each year, and about 60,000 died from it. In many developing countries, pneumonia remained a much greater problem, ranking first or second as a cause of death.

Who Is at Risk for Pneumonia?

Anyone can get pneumonia, but it tends to strike people whose natural defenses against infection are weak, especially the elderly and infants and young children. People with AIDS and organ transplant patients (who must take drugs to weaken the immune system) are also at higher risk, as are people living with diabetes * , chronic obstructive pulmonary disease (a lung disease), heart failure, tuberculosis * , sickle cell anemia * , or another serious illness.

Pneumonia is a common infection among people who are hospitalized for something else, especially people recovering from surgery and those who are placed on breathing machines (ventilators). Hospital-acquired pneumonia is almost always bacterial and is often caused by strains * of bacteria that are resistant to many antibiotics. For this reason, and because it affects people who are already sick, hospital-acquired pneumonia is more frequently serious or fatal compared to cases of pneumonia acquired outside the hospital.

What Happens When People Have Pneumonia?


Symptoms of pneumonia vary depending on the cause of the pneumonia and the health of the person with the infection. Symptoms of bacterial pneumonia may appear suddenly and include high fever; chills; rapid breathing; a deep cough that brings up greenish mucus, which is sometimes mixed with blood; and severe chest pain that worsens with breathing and coughing. Other symptoms include a bad headache, loss of appetite, tiredness, nausea, and vomiting.

People with atypical or mycoplasma pneumonia often get a persistent dry cough, sore throat, skin rash, and muscle and joint pain. Because these are not the classic symptoms of pneumonia, people may think they just have a mild case of flu.

Various physician organizations have developed recommendations for the diagnosis and treatment of many diseases, including pneumonia. The pneumonia guideline of the Infectious Diseases Society of America, for example, describes the specific laboratory, x-ray, and other tests that should be performed to diagnose pneumonia; lists the specific drugs that should be used to treat different types of pneumonia; and gives specific benchmarks for when it is safe for a patient recovering from severe pneumonia to leave the hospital. Guidelines such as these are developed by groups of medical experts who carefully review the latest medical research. By following this type of best-practice guideline, doctors can be confident they are giving their patients up-to-date and effective medical care.

Viral pneumonia can produce symptoms similar to those of the flu, including fever, muscle aches, breathlessness, rapid breathing, and a dry cough, which can worsen and eventually bring up a small amount of mucus.

People who are elderly or who have immune system problems often have milder symptoms in the beginning, even though their illness may be more dangerous. They might, for instance, have just a low-grade fever, tiredness, or confusion, and a sense of being ill.


To identify the infection, the doctor first asks questions about the patient's symptoms and then listens to a patient's chest with a stethoscope. Fluid in the lungs often produces a crackling sound when a person breathes, which can point to pneumonia. Sometimes the doctor will not be able to hear any air moving through the affected part of the lung. Chest x-rays may also be taken, and a cloudy, dense-appearing area may be seen in one or both lungs, particularly in a person who has bacterial pneumonia.

Next, the doctor may take samples of blood and sputum (SPYOO-tum, coughed-up mucus) to try to find out what is causing the pneumonia. These samples can be examined under a microscope and sent to the lab for culture * and identification of the organism causing the infection. In severe cases, a bronchoscopy * or a lung biopsy * may be performed.


If a specific type of bacteria has been identified as the cause of the pneumonia, the doctor can prescribe antibiotic drugs that target those bacteria. If the germ is not identified but bacteria are suspected, the doctor may give antibiotics that are active against the most likely bacteria, which is called empirical treatment. If the cause is a virus or fungus, antibiotics will not help. Instead, antiviral and antifungal drugs may be prescribed, although not all viruses have treatments.

When the pneumonia is severe, people are often hospitalized, particularly if they are in danger of dehydration * , need more oxygen, or cannot breathe well enough on their own.

Two medical organizations, the Infectious Diseases Society of America and the American Thoracic Society, jointly developed guidelines for physicians on the best methods for diagnosing and treating pneumonia. The guidelines are revised every few years to reflect current medical research.

People with pneumonia get relief from symptoms and aid their recovery by:

With treatment, bacterial pneumonia symptoms usually improve within a day or two and are gone in one to two weeks, but symptoms of viral pneumonia and mycoplasma pneumonia can last longer.

How Is Pneumonia Prevented?

The pneumococcal (nu-mo-KAH-kal) vaccine * works well against pneumococcal bacteria that cause pneumonia and is recommended for people over age 65 and children under age 2, those with certain other illnesses, or those with weakened immune systems. The vaccine typically needs to be given only once in a lifetime. Yearly flu vaccines can help prevent pneumonia caused by influenza viruses. Because pneumonia often is caused by contagious respiratory infections, people can protect themselves further by avoiding being near people who are sick; hand washing; not touching used tissues; and never sharing food, drinks, or eating utensils.

See also AIDS and HIV Infection • Bacterial Infections • Chlamydial Infections • Common Cold • Fungal Infections • Influenza • Legionnaires’ Disease • Mycoplasma Infections • Streptococcal Infections • Tuberculosis • Vaccines and Immunization • Viral Infections


Books and Articles

Metersky, Mark L., Robert G. Masterton, Hartmut Lode, Thomas M. File Jr., and Timothy Babinchak. “Epidemiology, Microbiology, and Treatment Considerations for Bacterial Pneumonia Complicating Influenza.” International Journal of Infectious Diseases 16, 5 (May 2012): e321–e331. (accessed November 11, 2015).

Sharlach, Molly. “Pneumonia-Causing Bacteria Poke Holes in Heart.” The Scientist. September 18, 2014. (accessed August 12, 2015).


American Lung Association. “Pneumonia.” (accessed November 11, 2015).

Centers for Disease Control and Prevention. “Pneumonia.” (accessed November 11, 2015).

MedlinePlus. “Pneumonia.” U.S. National Library of Medicine. (accessed August 12, 2015).

Merck Manual: Consumer Version. “Pneumonia.” (accessed November 11, 2015).


American Lung Association. 55 W. Wacker Dr., Suite 1150, Chicago, IL 60601. Toll-free: 800-LUNG-USA. Website: (accessed August 12, 2015).

Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: (accessed August 11, 2015).

World Health Organization. Avenue Appia 20, CH 1211, Geneva 27, Switzerland. Telephone: 41-22-791-21-11. Website: (accessed August 25, 2015).

* oxygen (OK-si-jen) is an odorless, colorless gas essential for the human body. It is taken in through the lungs and delivered to the body by the bloodstream.

* mucus (MYOO-kus) is a thick, slippery substance that lines the insides of many body parts.

* virulent comes from the Latin word for poisonous, and describes a microbe that is especially well suited to countering the immune system.

* respiratory tract, also called the respiratory system, includes the nose, mouth, throat, and lungs. It is the pathway through which air and gases are transported down into the lungs and back out of the body.

* intestines are the muscular tubes that food passes through during digestion after it exits the stomach.

* bladder (BLAD-er) is the sac that stores urine produced by the kidneys prior to discharge from the body.

* croup (KROOP) is an infection involving the trachea (windpipe) and larynx (voice box) that typically occurs in childhood. It causes inflammation and narrowing of the upper airway, sometimes making it difficult to breathe. The characteristic symptom is a barking cough.

* AIDS, or Acquired Immunodeficiency (ih-myoo-no-dih-FIH-shensee) Syndrome, is an infection that severely weakens the immune system; it is caused by the human immunodeficiency virus (HIV).

* diabetes (dye-uh-BEE-teez) is a condition in which the body's pancreas does not produce enough insulin or the body cannot use the insulin it makes effectively, resulting in increased levels of sugar in the blood. This can lead to increased urination, dehydration, weight loss, weakness, and a number of other symptoms and complications related to chemical imbalances within the body.

* tuberculosis (too-ber-kyoo-LOsis) is a bacterial infection that primarily attacks the lungs but can spread to other parts of the body.

* sickle cell anemia also called sickle cell disease, is a hereditary condition in which the red blood cells, which are usually round, take on an abnormal crescent shape and have a decreased ability to carry oxygen throughout the body.

* strains are various subtypes of organisms, such as viruses or bacteria.

* culture (KUL-chur) is a test in which a sample of fluid or tissue from the body is placed in a dish containing material that supports the growth of certain organisms. Typically, within days the organisms will grow and can be identified.

* bronchoscopy (brong-KOS-kopee) is a procedure used to examine the bronchi, the major air passages in the lungs, with an instrument called a bronchoscope, which is a tool for looking inside the lungs that is made up of a lighted tube with viewing lenses. A bronchoscope has channels through which samples of material can be taken from the lungs for study in the laboratory.

* biopsy (BI-op-see) is a test in which a small sample of skin or other body tissue is removed and examined for signs of disease.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

* vaccine (vak-SEEN) is a preparation of killed or weakened germs, or a part of a germ or product it produces, given to prevent or lessen the severity of the disease that can result if a person is exposed to the germ itself. Use of vaccines for this purpose is called immunization.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)