The etiological agent of Primary Atypical Pneumonia (PAP) is Mycoplasma pneumoniae.

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Primary atypical pneumonia presents with a more scattered, interstitial-type involvement and a gradual onset, often in younger adults. The classic etiologic agent is Mycoplasma pneumoniae, a tiny bacterium that lacks a cell wall and therefore does not Gram-stain. This cell-wall absence explains its resistance to beta-lactam antibiotics and its characteristic presentation as a “walking pneumonia” with a dry cough rather than a typical, lobar consolidation. While Chlamydia pneumoniae can also cause atypical pneumonia, Mycoplasma pneumoniae remains the most iconic association with PAP. In contrast, Streptococcus pneumoniae usually causes typical lobar pneumonia with abrupt onset and productive sputum, and Klebsiella pneumoniae leads to severe, often necrotizing pneumonia with thick sputum, particularly in older or debilitated patients.