• Also Known As:
  • Lower Respiratory Tract Infection
  • Community-Acquired Pneumonia
  • Hospital-Acquired Pneumonia
  • Ventilator-Acquired Pneumonia
  • Healthcare-Associated Pneumonia
  • Walking Pneumonia
  • Double Pneumonia
  • Lobar Pneumonia
  • Atypical Pneumonia
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What is pneumonia?

Pneumonia is an infection of the lower respiratory tract caused by bacteria, viruses, or fungi. The infection can cause symptoms ranging from moderate to life-threatening. According to the Centers for Disease Control and Prevention (CDC), more than one million children and adults in the U.S. are admitted to a hospital for pneumonia each year. Worldwide, pneumonia kills more people than any other infectious disease, about 2.7 million people a year, and is the leading cause of death in children under 5 years old, according to the World Health Organization. Pneumonia can occur at any time of the year, but the greatest numbers of cases are seen seasonally during influenza (flu) season, typically late fall through early spring.

Although anyone can get pneumonia, it occurs most frequently in those who are very young or over the age of 65 or people with weakened immune systems (immune compromised). Pregnant women and people with an underlying condition such as cystic fibrosis or damaged lungs from smoking are also at higher risk for pneumonia. This also includes people who are on mechanical ventilation or chemotherapy or have received an organ donation. People with AIDS are extremely susceptible to pneumonia.

Bacteria, viruses, and fungi are microbes that can cause lower respiratory tract infections. Pneumonia is most often caused by a microbe that has slipped past the body’s immune defenses and taken up residence deep within the lungs in tiny structures called the alveolar sacs. Bacteria are the most common cause and viruses are the second most common cause.

These microbes may be spread to others through microscopic droplets of respiratory secretions that become airborne when an infected person coughs or sneezes. Airborne droplets can be directly inhaled by another person or settle on surfaces such as countertops, keyboards, and phone receivers in the vicinity. The microbe is spread when someone else touches the contaminated surface and then touches his or her own eyes or mouth or other mucous membranes. Pneumonia can also come from the normal bacteria in saliva, food or vomit when a person chokes or gags and accidentally breathes in their own fluids.

It takes more than exposure to a potential pathogen to cause pneumonia. Microbes are a constant presence in the environment, and people encounter sources of infection on a daily basis. In most cases, the lungs can handle the assaults. The respiratory system has layers of immune defense starting with the mucous and hair-like structures that trap and remove many pathogens from airways. Pathogens that make it past these initial defenses are then subject to attack by an array of immune system cells that identify and engulf pathogens identified as foreign to the body. Pneumonia occurs when these defenses are weakened or damaged or when the invading pathogens are virulent enough to overcome or evade them.

A wide range of viruses, bacteria, and (less commonly) fungi can cause pneumonia, but the majority of cases are due to just a few of these. The most likely microbes causing a specific case of pneumonia depends upon the age and health status of the affected person and to some degree on the time of year. Those with weakened immune systems and those who have traveled to specific regions of the world may develop pneumonia that is due to more unusual microbes.

The following terms that may be familiar describe some general kinds of pneumonia:

  • Walking pneumonia refers to a mild form of the disease that typically does not require bed rest. It is often caused by a virus or Mycoplasma pneumoniae.
  • Lobar pneumonia describes a form of the disease that affects one part (lobe) of a lung.
  • Double pneumonia is an infection that affects both lungs.
  • Aspiration pneumonia, or pneumonia caused by inhaling one’s own gastric juices or food particles, can be more difficult to treat. The pathogens that cause the infection may be harder to identify in these cases, so health practitioners may need to do more testing in order to prescribe the correct antibiotic. People with gastroesophageal reflux disease (GERD) or individuals who have trouble swallowing due to other conditions are more at risk for this type of pneumonia. Aspiration pneumonia is sometimes called anaerobic pneumonia because the bacteria that cause it are usually beneficial bacteria that live in our digestive tract and do not require oxygen to live.

The setting where pneumonia was acquired will determine the likely cause of the pneumonia, how to best prevent the spread of the infection, and guide the appropriate treatment.

  • Community-acquired pneumonia—when a person becomes infected during daily activities outside the healthcare setting
  • Hospital-acquired pneumonia—when an infection occurs while a person in the hospital for another illness
  • Ventilator-acquired pneumonia–occurs while a person is connected to a ventilator machine
  • Healthcare-associated pneumonia—when a person is infected while in a healthcare-associated setting, such as a nursing home or dialysis clinic

Hospital-acquired and healthcare-associated microbes are more likely to be resistant to first-line antimicrobials. Community-acquired pneumonia is more likely to be due to bacteria that are susceptible to commonly prescribed antibiotics or due to seasonal viruses for which antimicrobial drugs are not the appropriate treatment.


Causes of Pneumonia


Bacteria are the most common cause of pneumonia in adults but are uncommon in young children. There are many different types of bacteria that can cause pneumonia, some more common than others. Sometimes people, particularly those who are elderly or who have other health problems, will develop bacterial pneumonia after having a viral respiratory infection.

Some bacterial pneumonias are relatively rare but may be seen in certain populations or in certain geographic areas. Travelers and people who have immigrated from certain countries may become infected with bacteria that are less common in the U.S. Also, those with pre-existing conditions such as decreased lung function or weakened immune systems may be more vulnerable to types of bacteria not found in the general population.

The most common cause of bacterial pneumonia contracted outside of a hospital or healthcare setting (community-acquired) is:

  • Streptococcus pneumoniae—these bacteria cause pneumococcal pneumonia and are easily spread through normal social contact; there are many serotypes of S. pneumoniae so you may get repeat infections with different types; a vaccine is available for the most common serotypes of S. pneumoniae to help prevent pneumonia and other serious infections, including meningitis and blood infections (septicemia)

Other less common causes include:

  • Haemophilus influenzae type B (Hib)—most often causes pneumonia but can cause other invasive infections such as meningitis and septicemia; while, infections are now less common due to Hib vaccinations, infections may occur with other types of H. influenzae bacteria that are not covered by the Hib vaccine
  • Moraxella catarrhalis—more typically a source of middle ear infections in children
  • Staphylococcus aureus—has become more common in hospital settings recently from drug-resistant strains like methicillin-resistant S. aureus (MRSA) or following an influenza (flu) infection
  • Klebsiella pneumoniae— can cause pneumonia in people already weakened by disease, usually in a hospital setting
  • Streptococcus agalactiae (Group B strep)—typically colonizes the female vagina and can be passed from a pregnant woman to her baby during childbirth
  • Pseudomonas aeruginosa—often affects those with decreased lung function due to cystic fibrosis

Atypical pneumonias are bacterial lung infections that do not respond to commonly prescribed antibiotics. Examples include:

  • Mycoplasma pneumoniae—a common cause of upper respiratory infections, with an estimated 2 million cases in the U.S. each year; responsible for 15-20% of cases of community-acquired pneumonia. It causes a less severe infection known as “walking pneumonia.” It typically affects people younger than age 40.
  • Chlamydophila pneumoniae—causes about 10% of the community acquired forms of pneumonia and most commonly occurs in adults ages 65 to 79.
  • Legionella pneumophila—causes Legionnaires’ disease; infections occur when an individual inhales steam, mist, or airborne droplets contaminated with the bacteria. The infection is not spread from one person to another. A legionella infection often requires hospitalization and has a significant death rate.

Mycobacterium tuberculosis, which causes tuberculosis (TB), and nontuberculous Mycobacteria (M. avium-intracellulare, M. kansasii and other species of mycobacteria) are less common in the U.S., but rates of infection are rising in other countries. Cases in the U.S. may be seen, for example, in travelers, prison populations, and those with AIDS.


Viruses that infect the respiratory system cause about one-third of all cases of pneumonia in the U.S. each year and are the most common cause of pneumonia in infants, accounting for approximately 90% of all lower respiratory infections. Less than 20% of pneumonias in adults younger than 60 years old are viral. In those over 60-65 years of age, viral causes become increasingly common.

Viral pneumonia is often mild, requiring only supportive care. However, more serious cases may require hospitalization. Sometimes secondary bacterial pneumonia may develop following a primary viral infection.

The most common causes of viral pneumonia are:

  • Influenza virus—the most common cause of viral pneumonia in adults
  • Parainfluenza virus—commonly causes respiratory infections in infants and young children and can progress to pneumonia
  • Respiratory syncytial virus (RSV)—the most common cause in children younger than one year old
  • Adenoviruses–common viruses that often cause respiratory illness; usually the infections are not serious, but they may be serious in people with weakened immune systems

Other less common causes include:

  • Adenoviruses—common viruses that often cause respiratory illness but usually are not serious
  • Human metapneumovirus—this virus was first identified in 2001; in children, it is thought to cause about 5%-15% of hospitalizations due to lung infections.
  • Cytomegalovirus (CMV)—may cause serious physical and developmental problems when women are infected for the first time (primary infection) during pregnancy and then pass the infection to the developing baby across the placenta.
  • Measles—also called rubeola, is an extremely contagious viral infection that is transmitted through respiratory secretions. Most people recover within a couple of weeks, but up to 20% develop complications that may include pneumonia.
  • Varicella (chickenpox)—caused by an infection with the varicella zoster virus (VZV), a member of the herpes virus family; most cases resolve without complications. Pneumonia is an uncommon complication.
  • Rhinoviruses—the viruses causing the common cold in children and adults; may occasionally infect the lower respiratory tract and cause pneumonia in young children and the elderly


Pneumonias caused by fungi are relatively rare. Some fungi cause pneumonia only in people who have weakened immune systems. These people include organ transplant recipients who are on immunosuppressive drugs, people on chemotherapy regimens, and those with AIDS. These fungi are referred to as “opportunistic.” Examples include:

  • Pneumocystis jiroveci—the most common in the U.S. in people with AIDS; it was previously known as P. carinii; pneumonia caused by this organism may still sometimes be called Pneumocystis carinii pneumonia or PCP.
  • Aspergillus species—can cause aspergillosis and fungal masses in the sinuses and lungs
  • Candida species—these yeasts are part of the normal human flora; infections can occur in the moist mucous membranes of the body.
  • Cryptococcus neoformans—anyone may become infected, but the highest prevalence in the U.S. is in people who have HIV/AIDS

Some fungi are pathogenic meaning they can cause pneumonia in people regardless of their health status. People become infected when they breathe in fungal spores. These types are usually found only in certain areas of the U.S. and/or the world:

  • Histoplasma capsulatum—causes histoplasmosis and is found in soil contaminated with bat or bird droppings; typically found in Ohio and Mississippi river valleys
  • Coccidioides immitis— causes coccidioidomycosis, also called Valley Fever; often found in Arizona and parts of California
  • Blastomyces dermatididis— found in south central, southeastern, and midwestern U.S. in moist soil and decaying wood or leaves

For more on these, read the article on Fungal Infections and Fungal Tests.

Unusual and serious causes

Some exotic and rare causes of pneumonia may infect otherwise healthy individuals and can be fatal. Examples include:

  • Hantavirus—contracted from infected rodent droppings. Most infections occur in the western part of the U.S. In 2016, the CDC reported 21 pulmonary hantavirus infections and a fatality rate of 45%
  • The bacteria Bacillus anthracis (anthrax)—an uncommon cause of pneumonia; due to exposure to a natural source such as infected animals or animal products
  • The bacteria Yersinia pestis (pneumonic plague)—infection may occur in the western U.S. by contact with an infected animal or by breathing in droplets coughed up by a person infected with plague or when bubonic or septicemic plague spreads to the lungs
  • Middle East Respiratory Syndrome Coronavirus (MERS-CoV)—this newly discovered (2012) respiratory virus has caused serious lower respiratory infections in some people who live in or traveled to countries in the Arabian Peninsula. About one-third of infected individuals have died from the disease. As of 2014, however, there is no evidence that the virus spreads easily within communities and most cases of person-to-person spread have occurred in healthcare workers and other close contacts, such as family members and caregivers of people sick with MERS.

About Pneumonia

Signs, Symptoms and Complications

Pneumonia symptoms vary, depending on the age and health status of the person affected and on what microbe caused the infection. Very young infants may grunt and wheeze, breathe rapidly, and be irritable and lethargic. The elderly may experience confusion. Bacterial pneumonia may emerge following a viral infection and may just be seen as a lingering or worsening cold or bout of the flu.

Common pneumonia symptoms include:

  • Cough
  • Fever and chills
  • Fatigue
  • Shortness of breath
  • Headache
  • Muscle aches
  • Chest pain
  • Nausea and vomiting
  • Feeling worse after a cold or the flu

Complications that can occur with pneumonia include the buildup of fluid in the lungs, the scarring of lung tissue (which can lead to recurrent infections), respiratory failure, sepsis, and, rarely, lung abscesses (pockets of pus in lung tissue).


The goals with testing are to determine the microbe causing the pneumonia, to limit its spread to other people, to determine the severity of the pneumonia, and to guide treatment. In a significant number of cases, it is not possible to determine the exact cause of acute pneumonia, and treatment is based upon the person’s clinical presentation and medical history, the health practitioner’s experience, likely microorganisms present in the community at the time, such as influenza, and on established guidelines for the care of lower respiratory infections. If a person’s symptoms do not resolve, then additional testing may be performed to help diagnose less common pneumonia causes.

Non-laboratory Tests
The search for the cause of symptoms typically begins with a physical exam. As part of the exam, a health practitioner listens to a person’s lungs for abnormal breath sounds that may indicate a lung infection. This is often followed with imaging studies. These may include:

  • Chest X-ray (radiography)—used to detect and help evaluate the severity of a lung infection
  • CT (computed tomography) scan—sometimes use to detect and evaluate a lung infection and to look for other non-infectious causes of a person’s symptoms
  • Bronchoscopy—a procedure used to look inside the lungs with a thin, flexible camera-tipped tube. This procedure may be done if treatment is not working well.

Laboratory Tests
Depending on the affected person’s medical history and the signs and symptoms that are present at the time of the physical exam, a number of laboratory tests may be performed to help make a diagnosis.

General laboratory tests may include:

  • CBC (complete blood count)—evaluates the type and number of white blood cells; results may indicate that an infection is present
  • BMP (basic metabolic panel)—blood tests for sodium, potassium, and other chemistries to help determine the severity of the illness
  • Blood gases—measures the pH and the amount of oxygen (O2) and carbon dioxide (CO2) present in a sample of blood, usually from an artery, in order to evaluate lung function

Examples of tests for suspected bacterial pneumonia include:

Examples of tests for suspected viral pneumonia include:

  • Influenza tests—used to help diagnose an influenza infection (flu) and sometimes to help document the presence of influenza in the community; testing may also be used to identify the type and/or the strain of influenza causing the infection.
  • RSV testing—used during the RSV season to help diagnose the infection in people who have moderate to severe symptoms and lower respiratory tract involvement. It is primarily ordered on infants (between the ages of six months and two years), the elderly, and those with compromised immune systems, such as those who have pre-existing lung disease or who have had an organ transplant.
  • Viral culture—used to grow a broad range of viruses that may cause infections

Fungal tests—ordered when a fungal infection is suspected. Example include:

  • Fungal cultures—used to identify the specific fungi present. Many fungi are slow-growing, so it may take weeks for results. Susceptibility testing performed on fungi isolated from a culture can help determine which antifungal is best to use for treatment.
  • Tests for fungal antigens and antibodies—used to determine if an individual has, or recently had, a specific fungal infection. They are more rapid than fungal cultures but only test for a specific fungal species, so the healthcare practitioner has to know what fungal organism to test for. Many people have fungal antibodies from a prior exposure to the fungus, so a single antibody test may not confirm the presence of a current infection. Sometimes, blood samples are collected 2 to 3 weeks apart (acute and convalescent samples) and tested to see if antibody levels (titers) are changing; the evaluation of these results may take several weeks.

Molecular tests—these tests may be used to identify the presence of bacteria, fungi and/or viruses in a respiratory sample by detecting the genetic material (DNA, RNA) of the microbes. These types of tests are generally very sensitive and specific and are particularly useful in detecting microorganisms that are difficult to culture.

  • Some tests look for a single specific microbe, such as Mycoplasma pneumoniae. A common method is polymerase chain reaction (PCR).
  • Newer methods can simultaneously test for several bacteria, fungi and/or viruses. Examples of these methods include multiplexed PCR and DNA microarrays.

Pleural fluid analysis—if fluid has accumulated in the space between the lungs and chest wall, the fluid may be tested to help determine the cause of infection.

Prevention and Treatment

Individuals can protect themselves against exposure to the bacteria, viruses, and fungi that cause pneumonia by following good hygiene practices. Such practices include:

  • Frequent and thorough hand washing
  • Respiratory etiquette of coughing or sneezing into a tissue, elbow or sleeve
  • Using disinfectant to clean surfaces that are regularly touched by hands, such as door knobs, handles, keyboards, remotes, and other devices
  • Avoiding touching one’s face, eyes, nose, and mouth without washing hands
  • Avoiding close contact with people ill with respiratory infections

Several vaccines are available to help protect against or decrease the risk of certain causes of pneumonia, including Streptococcus pneumoniae (pneumococcal), Haemophilus influenzae type b (Hib), seasonal influenza, and several others. People should talk to their health practitioners about these options.

Bacterial and mycoplasma pneumonias are typically treated with antibiotics. Fungal infections are treated with antifungal medications. Some infections may require treatment for an extended period of time.

Depending on the cause of the viral pneumonia and the duration of symptoms before seeing a health practitioner, antiviral medications may be given, but often only supportive care is an option.

People with severe cases of pneumonia may require hospitalization, treatment with oxygen or other breathing assistance, and intravenous antimicrobial drugs.

View Sources

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