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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).
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 decreased lung function due to Cystic Fibrosis (CF).
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)

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