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Overview of Pneumonia


Sanjay Sethi

, MD, University at Buffalo, Jacobs School of Medicine and Biomedical Sciences

Reviewed/Revised Sep 2022
Topic Resources

Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them.

  • Pneumonia is one of the most common causes of death worldwide.

  • Often, pneumonia is the final illness that causes death in people who have other serious, chronic diseases.

  • Some types of pneumonia can be prevented by immunization.

In the United States, about 4 to 5 million people develop pneumonia each year (excluding pneumonia caused by COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more ), and 55,000 of them die. In the United States, pneumonia, including pneumonia that occurs along with influenza Influenza (Flu) Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more , is the eighth leading cause of death and is the leading infectious cause of death. Pneumonia is the most common cause of death among infections that develop while people are hospitalized and is the most common overall cause of death in medically underserved countries. Pneumonia is also one of the most common serious infections in children and infants.

Causes of Pneumonia

Pneumonia is caused by different microorganisms—including bacteria Overview of Bacteria Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more , viruses Overview of Viral Infections A virus is composed of nucleic acid, either DNA or RNA, surrounded by a protein coat. It requires a living cell in which to multiply. A viral infection can lead to a spectrum of symptoms from... read more , mycobacteria Infections Caused by Bacteria Related to Tuberculosis (TB) Many species of mycobacteria exist. The species Mycobacterium tuberculosis is the one that causes tuberculosis. The other mycobacteria species that cause disease are discussed here. These... read more , fungi Overview of Fungal Infections Fungi are living organisms, but they are not plants or animals. All living things are divided into categories called kingdoms, and fungi have their own kingdom. Some fungi cause infections in... read more , and parasites Overview of Parasitic Infections A parasite is an organism that lives on or inside another organism (the host) and benefits (for example, by getting nutrients) from the host at the host's expense. Although this definition actually... read more . Bacterial and viral pneumonias are much more common than mycobacterial, fungal, or parasitic pneumonias. The specific organisms vary depending on the person's age, health, location, and other factors. More than one microorganism may be involved. For example, influenza Influenza (Flu) Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more (a viral infection) is often complicated by bacterial pneumonia.

  • The cough reflex, which helps expel mucus and foreign substances

  • The cells lining the lung passageways, which prevent microorganisms from invading the lungs and which push mucus and foreign substances upward so that they can be coughed out

  • Proteins made by lung cells, which attack microorganisms

  • White blood cells within the lungs, which are part of the normal immune system and which also attack microorganisms

Pneumonia develops when

  • Defense mechanisms are not functioning correctly.

  • A large amount of bacteria is inhaled and overwhelms normal defenses.

  • A particularly infective organism is introduced.

Usually pneumonia starts after microorganisms are inhaled (aspirated) from the upper airways into the lungs, but sometimes the infection is caused by an imbalance in the microorganisms in the airways and lungs or by microorganisms that are inhaled from the air, carried to the lungs by the bloodstream, or invade the lungs directly from a nearby site of infection.

Categories of pneumonia

Where people are when pneumonia develops is important because different organisms tend to be present in different settings. Organisms in some settings, such as hospitals, are typically more dangerous and more commonly resistant to antibiotics than organisms present in other settings. Also, people in some settings are more likely to have disorders that make them more likely to develop pneumonia. Some categories of pneumonia include

Health care–associated pneumonia, which is an infection acquired in a health care setting other than the hospital, such as a nursing home or dialysis center, is considered a subset of community-acquired pneumonia because these people are likely to have pneumonia that is caused by the same organisms that are likely to infect other people living in the community .

Other categories of pneumonia include

  • Obstructive pneumonia, which occurs when a blockage of the air passages in the lungs (such as caused by a tumor) causes bacteria to accumulate behind the blockage

"Walking" pneumonia is a nonmedical term used to describe a mild case of community-acquired pneumonia Community-Acquired Pneumonia Community-acquired pneumonia is lung infection that develops in people who are not patients in a hospital, usually in people with normal (competent) immune systems, or in those who are immunocompromised... read more that does not require bedrest or hospitalization. Some people might even feel well enough to go to work and participate in other daily activities.

Risk factors for pneumonia

Pneumonia may occur after surgery, particularly abdominal surgery, or after an injury (trauma), particularly a chest injury, because the pain of those conditions keeps people from breathing deeply and from coughing. If people do not breathe deeply and cough, microorganisms are more likely to remain in the lungs and cause infection. Other people who do not breathe deeply and cough include those who are debilitated, bedridden, paralyzed, or unconscious. Such people are also at risk of pneumonia.

Another critical feature is whether the pneumonia occurs in a healthy person or in someone who has an impaired immune system Pneumonia in Immunocompromised People Pneumonia is infection of the lungs. Pneumonia in people whose immune system is weakened or impaired (for example, by acquired immunodeficiency syndrome [AIDS], cancer, organ transplantation... read more . A person who has an impaired immune system is far more likely to contract pneumonia, including pneumonia caused by unusual bacteria and viruses and even fungi or parasites. Also, a person whose immune system is impaired may not respond as well to treatment as someone whose immune system is healthy. People who may have an impaired immune system include those who

  • Are being treated with certain drugs (such as corticosteroids or chemotherapy drugs)

  • Have certain diseases, such as acquired immunodeficiency syndrome (AIDS) or various types of cancer

  • Have an undeveloped immune system, as is the case of infants and toddlers

  • Have an immune system that is impaired by severe illness

Symptoms of Pneumonia

The most common symptom of pneumonia is

  • Cough that produces sputum (thick or discolored mucus)

Other common pneumonia symptoms include

  • Chest pain

  • Chills

  • Fever

  • Shortness of breath

These symptoms may vary, however, depending on how much of the lungs are infected and which organism is causing it.

Sometimes people who have pneumonia have digestive symptoms such as nausea, diarrhea, and loss of appetite (anorexia).

Symptoms vary even more in infants and older people. Fever may not occur. Chest pain may not occur, or people may not be able to communicate that they have chest pain. Sometimes the only symptom is rapid breathing or a sudden refusal to eat. Sometimes sudden confusion may be the only sign of pneumonia in an older person.

Complications of pneumonia

Common complications include

Severe pneumonia can prevent oxygen from getting to the bloodstream, causing people to feel short of breath. Low levels of oxygen can be life threatening.

The microorganism causing the pneumonia can enter the bloodstream, or the body's response to the infection can be excessive, resulting in decreased blood pressure that can be life threatening, a condition called sepsis Sepsis and Septic Shock Sepsis is a serious bodywide response to bacteremia or another infection plus malfunction or failure of an essential system in the body. Septic shock is life-threatening low blood pressure ... read more .

Some pneumonias can lead to lung abscess Abscess in the Lungs A lung abscess is a pus-filled cavity in the lung surrounded by inflamed tissue and caused by an infection. A lung abscess is usually caused by bacteria that normally live in the mouth and are... read more or empyema. An abscess is a pocket of pus within tissue. A lung abscess forms when a small area of the lung dies and a collection of pus forms in its place. An empyema Types of fluid Types of fluid is a collection of pus in the space between the lung and the chest wall.

An overwhelming infection or excessive inflammation in response to the infection can cause severe injury to the lungs, which can manifest as ARDS. ARDS causes shortness of breath, usually with rapid, shallow breathing. People who have ARDS usually require breathing support with a mechanical ventilator Mechanical Ventilation Mechanical ventilation is use of a machine to aid the movement of air into and out of the lungs. Some people with respiratory failure need a mechanical ventilator (a machine that helps air get... read more for an extended amount of time.

Diagnosis of Pneumonia

  • Usually a chest x-ray, but sometimes a computed tomography (CT) scan of the chest

  • Sometimes testing to identify the microorganism causing the pneumonia

A doctor checks for pneumonia by listening to the chest with a stethoscope. Pneumonia usually produces distinctive sounds. These abnormal sounds are caused by narrowing or closing of the airways or filling of the normally air-filled parts of the lungs with inflammatory cells and fluid, a process called consolidation. In most cases, the diagnosis of pneumonia is confirmed with a chest x-ray but sometimes a CT scan of the chest is done. In mild cases doctors may decide to treat based on the symptoms and the results of the examination.

In people who are sick enough to require hospitalization, doctors often test specimens of sputum, blood, and urine in an attempt to identify the organism causing pneumonia. In very sick people, in people with a known problem of the immune system, or when looking for certain unusual organisms, doctors sometimes will obtain sputum samples by giving a vapor treatment that causes the person to cough deeply (inducing sputum production) or insert a bronchoscope Bronchoscopy Bronchoscopy is a direct visual examination of the voice box (larynx) and airways through a viewing tube (a bronchoscope). A bronchoscope, a thin viewing tube with a light, has a camera at the... read more Bronchoscopy (a small flexible tube equipped with a camera) into the airways. Sputum samples obtained by inducing a cough and particularly those obtained with a bronchoscope are less likely to contain saliva and are more likely than expectorated sputum samples to allow sputum cultures to identify the organism causing pneumonia.

It is particularly important for doctors to identify the organism that is causing pneumonia when people are severely ill, do not have a normal immune system, or are not responding well to treatment. However, despite these tests, the precise organism cannot be identified conclusively in most people who have pneumonia.

Prevention of Pneumonia

Deep-breathing exercises and therapy to remove mucus and secretions from the lungs help prevent pneumonia in people at high risk, such as those who have had chest or abdominal surgery and those who are debilitated.

Vaccines can help prevent pneumonia. Sometimes when an unvaccinated person has contact with a person known to have a virus that can cause pneumonia (such as influenza), doctors will prescribe certain antiviral drugs to try to prevent infection and pneumonia.

Vaccines to prevent pneumonia

Treatment of Pneumonia

  • Antibiotics and sometimes antiviral, antifungal, or antiparasitic drugs

  • Treatments to support breathing

People with pneumonia need to remove mucus and secretions from the lungs and may benefit from deep-breathing exercises Breathing exercises Chest physical therapy uses mechanical techniques, such as chest percussion, postural drainage, and vibration, to help clear secretions from the lungs. Respiratory therapists use several different... read more Breathing exercises . People with pneumonia who are short of breath or have low levels of oxygen in their blood are given oxygen, usually by a small plastic tube in the nostrils (nasal cannula). Although rest is an important part of treatment, complete bed rest can be harmful, and people are encouraged to move often and get out of bed and into a chair.

Usually antibiotics are started whenever bacterial pneumonia is suspected, even before the organism is identified. The prompt use of antibiotics reduces the severity of pneumonia and the chance of developing complications, some of which can lead to death.


When choosing an antibiotic, doctors consider which organism is likely to be the cause. Several factors may give clues to the organism causing pneumonia:

  • Type of pneumonia (community-acquired pneumonia, hospital-acquired pneumonia, obstructive pneumonia, aspiration pneumonia)

  • The person's age

  • Whether the person's immune system is functioning correctly

  • Whether the person has other lung diseases

  • Severity of the pneumonia

  • Whether the person has been given antibiotics by vein within the last 90 days

  • Information about what organisms are common in the local area and which antibiotics are able to kill them

  • Any available information from diagnostic testing, such as the identification of specific bacteria

In general, a doctor chooses an antibiotic that has "broader" activity, meaning that the antibiotic is effective against a wide range of microorganisms, even microorganisms that are resistant to some antibiotics, in the following circumstances:

  • When pneumonia is severe

  • When the person's immune system is not working correctly

  • When the person has hospital-acquired pneumonia, or other risk factors for developing pneumonia caused by a microorganism that is resistant to some antibiotics (for example, living in a nursing home and unable to do daily living activities or recent treatment with antibiotics)

Doctors may give a different antibiotic later, after the organism has been identified and its susceptibility to various antibiotics is known.

Of note, a "broad" antibiotic also kills the normal bacteria that live in the intestine and can result in a severe diarrhea that can be life-threatening, a condition called Clostridioides difficile-induced colitis Clostridioides (formerly Clostridium) difficile–Induced Colitis Clostridioides difficile (C. difficile)–induced colitis is an inflammation of the large intestine (colon) that results in diarrhea. The inflammation is caused by toxin produced... read more or antibiotic-associated colitis. Therefore, a "broad" antibiotic is used only in the circumstances described above. In comparison, for people with less severe pneumonia and for those in general good health, a "narrower" antibiotic is chosen that is usually appropriate for the most common microorganisms that cause pneumonia. Though these antibiotics can also cause diarrhea, it happens less frequently. These antibiotics are usually successful, and this approach decreases the risk of Clostridioides difficile-induced colitis, which is much more common with a broader antibiotic.

Antiviral and antifungal drugs

Antibiotics are not helpful for viral pneumonias. However, specific antiviral drugs are sometimes given if certain viral infections are suspected, such as influenza, COVID-19, or chickenpox. For influenza, specific antiviral drugs (such as oseltamivir or zanamivir) can reduce the duration and severity of illness if people begin taking the drugs within 48 hours of when symptoms start. However, once a person has developed influenza pneumonia, doctors are not sure whether the antiviral drugs will help, but they still usually give them. Often a bacterial pneumonia can develop after the viral infection. In this case, doctors give affected people antibiotics.

In rare cases, a fungus or parasite is the cause of the pneumonia, and an antifungal or antiparasitic drug is given.

Home versus hospital care

Often, people who have pneumonia but are not very sick can stay at home and take antibiotics by mouth. Older people, infants, and those who are short of breath, are very sick, or have preexisting heart or lung disease are usually hospitalized and given intravenous antibiotics, antivirals, or antifungals to start. Those antibiotics are usually switched to oral ones after a few days. People who need to be hospitalized may also need supplemental oxygen and intravenous fluids. People who are very sick may need to be sedated and temporarily put on a breathing machine Mechanical Ventilation Mechanical ventilation is use of a machine to aid the movement of air into and out of the lungs. Some people with respiratory failure need a mechanical ventilator (a machine that helps air get... read more (mechanical ventilator) that pushes air in and out of the lungs via a tube inserted in the throat.

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