Pneumonia describes an inflammatory condition affecting the alveoli of the lungs, in the vast majority of patients is due to a secondary bacterial infection

Aetiology

Community acquired pneumonia (CAP) may be caused by the following infectious agents:

• Streptococcus pneumoniae (accounts for around 80% of cases) - can cause reactivation of HSV resulting in cold sores
• Haemophilus influenzae (common in patients with COPD)
• Staphylococcus aureus: commonly after influenza infection
• Atypical pneumonias:
  • Mycoplasma pneumoniae - AHA and erythema multiforme may be seen
  • Legionella pneumophillia - commonly secondary to infected AC units
• Klebsiella pneumoniae is classically in alcoholics
• Pneumocystis Jiroveci - Typically in patients with HIV
• Causes of CAP in animal hide importers/sorters - Bacillus anthracis, coxiella burnettii
• Causes of CAP in exposure to birds - chlamydia psitacci (psittacosis), bats/bat dropping exposure, Histoplasma capsulatum (a fungus, classically affects cave-explorers)
• viruses - most commonly influenza A - can predispose staph A

Idiopathic interstitial pneumonia: a group of non-infective causes of pneumonia. Examples include cryptogenic organizing pneumonia which describes a form of bronchiolitis that may develop as a complication of rheumatoid arthritis or amiodarone therapy.

Classification:

• CAP: pneumonia developed outside of the hospital
• HAP: pneumonia occuring 48 hours or more after admission

*Causative organisms and 1st line Abx vary