Also Known As
Lower Respiratory Tract Infection
Community-Acquired Pneumonia
Hospital-Acquired Pneumonia
Healthcare-Associated Pneumonia
Walking pneumonia
Double pneumonia
Lobar pneumonia
Atypical Pneumonia
This article was last reviewed on
This article waslast modified on 5 April 2023.
What is it?

Pneumonia is an infection of the lungs, an invasion of lower respiratory tract by microrganisms that can cause a disease that ranges from mild to life-threatening. Worldwide, pneumonia kills more people than any other infectious disease, more than 4 million people a year, half of them under 5 years old.

Although anyone can get pneumonia, it is most common and potentially severe in those who are very young, over 65, immune compromised, or who have an another disease or condition that affects their lungs. This group of conditions includes people who have HIV/AIDS or have had an organ transplant and those who are on chemotherapy for cancer, are pregnant, in intensive care (ICU), on mechanical ventilation, have scarred or damaged lungs as occurs from smoking (COPD- chronic obstructive airways disease), or who have a lung disease such as cystic fibrosis or bronchiectasis or interstitial lung disease.

Pneumonia can occur at any time, but the greatest numbers of cases are seen in the late autumn through to early spring. Infection of the lungs can occur in five different ways:

  • Microorganisms can move down from the breathing airways into the lungs
  • Airborne droplets from a sneeze or cough can be inhaled
  • Surfaces contaminated with mucus or other respiratory secretions when touched by hands can spread through the mouth or eyes
  • Microorganisms in a person's own mouth or stomach can enter the lungs
  • An infection may spread from another site in the body, through the blood and into the lungs

It takes more than exposure to a potential disease-causing microorganism (pathogen) to get pneumonia. Microorganisms are always present in the environment, and we are exposed to infection daily. In most cases, the lungs can take care of these infections. The body has layers of defences that include mechanical barriers, protective microorganisms (normal flora), and the immune system. Pneumonia occurs when these defences are weakened or damaged and/or when the invading microorganisms are strong enough to overcome them.

A wide range of viruses, bacteria, and (less commonly) fungi or parasites can cause pneumonia, but the majority of cases are due to just a few of these. The most likely microorganism to cause pneumonia will depend upon the age and health of the person and on the time of the year. Those with poor immune systems and those who have been traveling abroard may develop pneumonia that is due to more unusual microorganisms.

Pneumonia may be grouped by the medical community into different categories. These distinctions are most useful in finding a cause of the pneumonia, how best to prevent its spread, and in guiding treatment.

  • Community-acquired pneumonia—when a person becomes infected during normal daily activities
  • Hospital-acquired pneumonia—when an infection occurs, for example, after surgery while connected to a ventilator or in an intensive care unit
  • Healthcare-associated pneumonia—when a person is infected from a healthcare establishment such as a nursing home or dialysis clinic

Hospital-acquired and healthcare-associated microorganisms are more likely to be resistant to first-line antibiotics where as 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 agents are not the appropriate treatment.

Accordion Title
About Pneumonia
  • Signs and Symptoms

    Pneumonia symptoms vary, depending on the age and health status of the person affected and on the microorganism causing the infection. Very young infants may grunt and wheeze, breathe rapidly, and be irritable and/or 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
    • Tiredness
    • Shortness of breath
    • Headache
    • Muscle aches
    • Chest pain
    • Nausea and vomiting

    Complications that can occur with pneumonia include the build-up 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).


  • Types

    Pneumonia can be due to a variety of bacteria and viruses, less commonly due to fungi, and rarely due to other microorganisms or parasites. Likely causes are associated with the age of the person, the season, the person's health status, and/or where the infection was contracted. Some of the more common examples of microorganisms that cause pneumonia are listed in the table below:

    Type Comments Some Causative Organisms
    Viral Viruses cause about one-third of all cases of pneumonia.
    They 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 admission to hospital. Sometimes bacterial pneumonia may develop following viral pneumonia.
    The most common causes of viral pneumonia are influenza parainfluenza, and respiratory syncytial virus (RSV) and COVID-19.
    Other less common causes include adenoviruses, human metapneumovirus, and cytomegalovirus (CMV).
    RSV is the most common cause in children younger than 1 year old.
    Bacterial Bacteria are the most common cause of pneumonia in adults. 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 an upper respiratory infection or the flu.
    Bacterial pneumonias in young children are uncommon.
    Atypical pneumonias are bacterial lung infections that do not respond to commonly prescribed antibiotics.
    Some bacterial pneumonias are relatively rare but may be seen in certain populations or in certain geographic areas. Travellers and people who have moved into the UK may be infected with bacteria that are less common in this country. Those with pre-existing conditions such as decreased lung function or weakened immune systems may be more likely to become infected from types of bacteria not found in the general population.
    Most commonly due to Community-acquired Streptococcus pneumoniae. Some other less common causes:
    Group B Streptococcus, Moraxella catarrhalis, Staphylococcus aureus, Haemophilus influenzae type B, Listeria monocytogenes, Escherichia coli, Klebsiella pneumoniae

    Atypical pneumonias:
    Mycoplasma pneumoniae
    Chlamydophila pneumoniae
    (Legionnaire's disease)
    Mycobacterium tuberculosis and nontuberculous Mycobacteria (M. avium-intracellulare, M. kansasii) may be seen, for example, in travellers, prison populations, and those with HIV/AIDS.
    Pseudomonas aeruginosa can affect those with damaged lungs such as bronchiectasis.
    Fungal Pneumonias caused by fungi are relatively rare.
    There are some types of fungi that usually cause pneumonia only in people who have weakened immune systems. These people include, for example, organ transplant recipients who are on immunosuppressant drugs, those on chemotherapy, and those with HIV/AIDS. These fungi are referred to as "opportunistic."
    Some types of fungi are pathogenic. This means that they can cause pneumonia in people regardless of their health status. These types are usually found only in certain areas of the US and/or the world.
    Some opportunistic fungi include: Pneumocystis jiroveci, Aspergillus species, Candida species, and Cryptococcus neoformans. P. jiroveci is the most common opportunistic infection in the UK in those with HIV/AIDS. It was previously known as P. carnii; thus pneumonia caused by this organism may still sometimes be called Pneumocystis carnii pneumonia or PCP.
    Pathogenic fungi include:
    Histoplasma capsulatum
    Coccidioides immitis
    Blastomyces dermatididis

    "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 M. pneumoniae. Lobar pneumonia describes a form of the disease that affects one lung, while double pneumonia is an infection that affects both lungs.

    Pneumonia due to the aspiration of oral or gastric secretions may be due to more than one microorganism and may include anaerobic bacteria. Some more exotic and rare causes of pneumonia may infect otherwise healthy individuals and can be fatal. Some of these include:

    • The systemic fungus Histoplasma capsulatum (histomplasmosis) — contracted from infected soil in caves
    • The hantavirus — contracted from infected rodent droppings
    • The fungus Coccidiodes immitis (coccidioidomycosis)
    • The bacteria Bacillus anthracis (anthrax) — due to a natural or bioterrorism exposure
    • The bacteria Yersinia pestis (pneumonic plague)


  • Tests

    The goals with testing are to find out what is the microorganism causing the pneumonia, to limit its spread to other people, to determine the severity of the pneumonia, and to guide treatment. It is often not possible to workout the exact cause of acute pneumonia, and treatment is then based symptoms, medical history, the doctor's experience, likely microorganisms present in the community at the time, and on established guidelines for the care of lower respiratory infections. If treatment is not working then tests may be performed to help diagnose less common causes of pneumonia.

    Non-laboratory Tests
    The search for the cause of symptoms typically begins with a physical examination. As part of this, a healthcare professional may listen to a person's lungs for abnormal rumbling, crackling, and bubbling noises that may indicate a lung infection. This is often followed with imaging studies. These may include:

    • Chest X-ray - used to detect and help evaluate the severity of lung infection
    • CT (Computed Tomography) scan — sometimes use to detect and evaluate lung infection and to look for other causes of a person's symptoms

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

    • FBC (Full Blood Count) — requested to evaluate the type and number of white blood cells
    • Urea & Electrolytes “U&E”s — blood tests for sodium, potassium, and other chemicals to help determine the severity of the illness
    • Arterial Blood gases — to also help determine the severity of the illness by assessing how much oxygen, carbon dioxide and hydrogen ion (pH) is in the blood.

    Tests for suspected bacterial pneumonia

    Tests for suspected viral pneumonia:

    • Polymerase chain reaction (PCR) — to detect a broad range of viral pathogens in respiratory secretions or a nose or throat swab.

    Other types of tests that may be performed depending on patient history and clinical presentation:

    • 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
    • Fungal culture — sometimes ordered when a fungal infection is suspected
    • Mycoplasma testing — blood test or special culture to help diagnose a mycoplasma infection
    • Legionella testing — urine test for the specific antigen; culture or molecular test on respiratory secretions; or blood sample for antibodies to diagnose a suspected Legionella infection


  • Prevention and Treatment

    Individuals can protect themselves from causes of pneumonia by following good hygiene practices. Such practices as frequent hand washing, coughing or sneezing into a tissue, elbow or sleeve, and cleaning surfaces that are regularly touched by hands prevent contact with these pathogens.

    Several vaccines are available to help protect against or decrease the risk of certain types of pneumonia, including pneumococcal, Haemophilus influenzae type b, seasonal influenza, and several others. People should talk to their doctors 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.

    Those with viral pneumonia are sometimes treated with antiviral medications but often are just given supportive care.

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