Pneumonia Types and Causes

There are different types of pneumonia that are classified according to :

  • mechanism of injury
  • area and extent of the inflammation (anatomical distribution)
  • setting in which the condition was acquired
  • microorganism involved in the infection

Types of Pneumonia

Broadly, pneumonia can be categorized as :

  1. Infectious pneumonia – this can be further categorized according to the causative microorganisms which is discussed below.
  2. Non-infectious pneumonia – this is caused by a number of mechanisms that trigger inflammation.
    • Chemical pneumonia
      • Inhalation of toxic gases or fumes
      • Aspiration of stomach acid, toxic chemicals (usually liquid) or solid particles like organic/inorganic dust
    • Aspiration pneumonia
    • This is chemical or infectious pneumonia that is a result of the following entering the lung :
      • Foreign objects
      • Gastric (stomach) contents
      • Aspirated fluids

Anatomical Distribution

  • Lobar Pneumonia
    • One of more lobes of the lung may be affected, with the infection being very localized to these areas.
    • Most cases are infectious and frequently arises as a secondary bacterial infection following a viral respiratory infection like influenza.
  • Bronchopneumonia
    • Affects the terminal parts of the airways (bronchi and bronchioles) with the inflammation involving small areas of lung tissue around these parts of the respiratory tract.
    • Often due to infectious causes complicating from acute bronchitis, tracheitis or influenza.
  • Interstitial Pneumonia
    • Affects the connective tissue and lining of the alveolar septa (walls between the air sacs) often resulting in widespread inflammation at multiple locations in the lung.
    • May be triggered by known infectious and non-infectious factors although the cause is often related to immune-mediated disorders. Frequently, no known cause can be identified (idiopathic).

Setting of Infection

The causes of pneumonia are usually due to an infection acquired within a clinical or institutional setting and often spread among patients or residents. Multi-drug resistant (MDR) bacteria are often implicated.

  • Community-acquired pneumonia (CAP) arises :
    • After an outpatient visit
    • Within 48 hours of hospital admission
  • Health Care-Associated Pneumonia (HCAP) includes :
    • Hospital-Acquired Pneumonia (HAP)
      • Develops after 48 hours or more of being admitted to hospital.
    • Ventilator-Associated Pneumonia (VAP)
      • Develops after 48 hours or more of being on a ventilator / endotracheal intubation or within 48 hours after being off a ventilator /extubation.
    • Institution-Acquired Pneumonia
      • Develops in patients within institutions, like mental health institutions, or residents of a nursing home (NHAP).

Pathogenic Microorgansims

Most cases of infectious pneumonia are bacterial in origin. Viral or fungal pneumonia may also occur with the latter often being associated with immunocompromised patients, particularly as opportunistic infections in HIV/AIDS patients.

  • Bacterial Pneumonia – common bacteria :
    • Streptococcus pneumoniae (pneumoccocal pneumonia)
    • Staphylococcus aureus
    • Enterococcus (Enterococcus faecalis, Enterococcus faecium)
    • Pseudomonas aeruginosa
    • Klebsiella pneumoniae **
    • Haemophilus influenzae
    • Escherichia coli **
    • Mycobacterium tuberculosis

** New strains may be NDM-1 superbug.

  • Viral Pneumonia – causative viruses :
    • Influenza including H5N1 (bird flu) and severe acute respiratory syndrome (SARS)
    • Parainfluenza
    • Adenovirus
    • Rhinovirus
    • Herpes simplex virus
    • Respiratory syncytial virus (RSV)
    • Cytomegalovirus (CMV)
  • Fungal Pneumonia – causative fungi :
    • Candida spp
    • Aspergillus spp
    • Mucor spp
    • Cryptococcus spp
    • Coccidioides spp
    • Blastomyces spp
  • Atypical pneumonia (walking pneumonia) includes less common microorganisms (mainly bacteria) that may cause pneumonia, which is usually mild.
    • Mycoplasma pneumoniae
    • Chlamydophila pneumoniae
    • Legionella pneumophila
    • Bordetella pertussis

Related Articles

  1. What is Pneumonia?
  2. Pneumonia Cough, Sputum and Other Signs and Symptoms


  1. Bacterial Pneumonia. Emedicine

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