• Legionella is the most common pathogen causing life-threatening atypical pneumonia.
• Micro-organisms involved in atypical pneumonia are capable of:
resisting phagocytosis (Mycoplasma pneumoniae);
creating abnormal phagosomes which do not fuse with secondary lysosomes (Legionella species);
surviving within phagocytes by inhibiting phagolysosomal fusion ( Chlamydia pneumoniae, Mycobacterium tuberculosis);
resisting lysosomal enzymes (Coxiella burnetii, Mycobacterium tuberculosis).
• The main defense against these infections is the cellular immune response.
• Essentially, any virus can produce any syndrome, but severe pneumonia in adults leading to admission to the intensive care unit is usually due to influenza and herpes viruses (varicella zoster virus).
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