Infection of the pulmonary parenchyma caused by various organisms.

  • Largest infectious cause of death worldwide.


Pneumonia was first described by Hippocrates (460–370 BC). The first descriptions of its clinical and pathological features were made 22 centuries later in 1819 by Laennec while Rokitansky in 1842 was the first to differentiate lobar and bronchopneumonia. During the next 47 years at least 28 terms were used to identify pneumonia , and by 1929 the total number of terms listed in the Manual of the International List of Causes of Death had grown to 94, with 12 sub-terms. The ICD-10 classification of diseases has removed some of the historical descriptive terms and ‘pneumonia’ is listed as the primary term in seven codes (J12–18) but it is also a descriptive term in seven other codes relevant to specific infectious and non-infectious aetiologies, times of life and complications of diseases and procedures. ICD-10 codes usually include subcategories so there are still many classifications for pneumonia. It is also of note that ‘other acute lower respiratory infections’ comprise three other codes (J20–22) for acute bronchitis, bronchiolitis and unspecified conditions.


Epidemiological classification:

  • Community-acquired pneumonia (CAP)
  • Healthcare-associated pneumonia (HAP)
    • Pneumonia that was not incubating at time of hospital admission and that presents clinically ≥ 48 hours (2 days) after hospitallization
  • Aspiration pneumonia
  • Atypical pneumonia

Anatomical classification:

  1. Bronchopneumonia
  2. Lobar pneumonia
  3. Interstitial pneumonia

Aetiological classification:

  • Bacterial pneumonia
  • Viral pneumonia
  • Fungal pneumonia
  • Atypical Pneumonia


Risk factors (adults):

  • Smoking
  • URTI
  • Alcohol
  • Corticosteroid therapy
  • Crowding
  • Indoor air pollution
  • Recent influenza infection
  • Pre-existing lung disease
  • HIV
  • Extremes of age

Risk factors (neonates & children):

  • Low birth weight
  • Large family size
  • Malnutrition
  • Vitamin A deficiency
  • Lack of breastfeeding
  • Family history of bronchitis

Organisms: Community-acquired pneumonia (CAP)

  • Typical CAP:
    • Streptococcus pneumoniae (pneumococcus) (gram-positive cocci) (M/C)
    • Haemophilus influenzae, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae
  • Atypical CAP:
    • Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella sp
    • Influenza virus, adenovirus, respiratory syncytial virus (RSV)
  • Aspiration pneumonia: Anaerobic bacteria

Organisms: Hospital-acquired pneumonia (HAP)

  • Staphylococcus aureus (gram-positive cocci) (M/C single organism)
  • Gram-negative bacilli (M/C group): Pseudomonas, Klebsiella, E. coli
  • Gram-negative bacteria: 50-80%
  • Gram-positive bacteria: 20-30%
The most commonly identified pathogens in patients with Hospital-Acquired Pneumonia HABP/VABP (SENTRY Study). | Cilloniz, C., Martin-Loeches, I., Garcia-Vidal, C., San Jose, A., & Torres, A. (2016). Microbial Etiology of Pneumonia: Epidemiology, Diagnosis and Resistance Patterns. International journal of molecular sciences, 17(12), 2120. doi:10.3390/ijms17122120


Pathological stages:

  • Stage I: Congestion & oedema
    • Seen in first 24 hours, proteinaceous exudate and bacteria present in alveoli
  • Stage 2: Red hepatization
    • RBCs and neutrophils seen in exudate
  • Stage: Gray hepatization
    • Corresponds to successful containment of infection.
    • RBCs lysed & degraded, abundant neutrophils & fibrin deposition
    • No bacteria detected
  • Stage 4: Resolution
    • Macrophages predominant
    • Debris of neutrophils, fibrin & bacteria cleared



Clinical features

Main symptoms of infectious pneumonia | By Mikael Häggström.When using this image in external works, it may be cited as:Häggström, Mikael. “Medical gallery of Mikael Häggström 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 20018762. Public Domain.orBy Mikael Häggström, used with permission. – All used images are in public domain., Public Domain,

Staphylococcal pneumonia:

  • Cavitation (also in Klebsiella, Pseudomonas, anaerobic bacteria, post-primary TB)
  • Pneumoatocoeles
  • Pleural effusion
  • Empyema
  • Abscess formation


  • Pleural effusion (20-40%) → complicated parapneumonic effusion or empyema (10%, M/C in pneumococcal pneumonia)



  • FBC (Full blood count)
    • ↑ (> 20 x 109/l) or ↓ (< 4 X 109/l) (MARKER)
    • Neutrophil leucocytosis
    • Haemolytic anaemia
      • Mycoplasma
  • Urea & Electrolytes
    • Urea > 7 mmol/l (~20 mg/dl) (MARKER)
    • Hyponatremia (MARKER)
  • LFT (Liver Function Test)
    • Abnormal
    • Hypoalbuminemia (MARKER)
  • Blood Culture
    • Bacteraemia (MARKER)
  • Serology
    • Acute and convalescent titres for Mycoplasma, Chlamydia, Legionella & viral infections
  • Cold agglutinins
    • Positive
      • 50% patients of Mycoplama
  • Arterial Blood Gas


  • Sputum samples
    • Gram stain, culture, antimicrobial sensitivity testing
  • Oropharynx swab
    • PCR (Atypical organisms)


  • Pneumococcal and/or Legionella antigen

X-ray (chest):

    • Patchy & segmental shadowing
  • Pneumococcal pneumoniaLobar consolidation
    • Patchy opacification → Homogenous consolidation (affected lobe)
    • Air bronchogram (air-filled bronchi appear luscent against consolidated lung tissue)
  • Staphylococcal pneumoniaPneumatoceles (thin-walled asymptomatic cysts)
  • Streptococcal pneumonia:
    • Interstitial pneumonia
    • Segmental involvement
    • Diffuse peribronchial densities/effusion (D/D Atypical Pneumonia)
  • Gram-negative pneumoniaMassive consolidation
  • Right upper lobe consolidation due to Klebsiella pneumoniae:
    • Bulging fissure sign: Expansive lobar consolidation causing fissural bulging or displacement by copious amounts of inflammatory exudate within the affected parenchyma
  • Silhouette sign: Loss of a normal lung–soft-tissue interface (loss of silhouette) caused by any pathologic mechanism that replaces or displaces air within the lung parenchyma.

CURB-65 criteria:

Symptom Points
Confusion 1
BUN >7 mmol/l 1
Respiratory rate ≥ 30 1
SBP < 90mmHg, DBP ≤ 60mmHg 1
Age ≥ 65 1


  • 0-1: Treat as an outpatient
  • 2: Hospital admission
  • 3-5: ICU admission

Differential diagnosis

  • Pulmonary infarction
  • Pulmonary/pleural TB
  • Pulmonary oedema
  • Pulmonary eosinophilia
  • Malignancy:
    • Bronchoalveolar Cell Carcinoma
  • Rare disorders:
    • Cytogenic organising pneumonia/broncholitis obliterans organising pneumonia


Community-acquired MRSA (CA-MRSA):

  • β-lactam (eg. ceftriaxone) + fluoroquinolone/azithromycin + linezolid/vancomycin  ± clindamycin


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