Localized, suppurative necrotizing process occurring within the pulmonary parenchyma
Microbiology
Community-acquired: anaerobes mostly aspiration, anaerobic bacteria from oral cavity: peptostreptococcus, prevotella, bacteroides, fusobacterium species
Hospital-acquired: Staph, E coli, Klebsiella, pseudomonas, legionella
Community-acquired: anaerobes mostly aspiration, anaerobic bacteria from oral cavity: peptostreptococcus, prevotella, bacteroides, fusobacterium species
Hospital-acquired: Staph, E coli, Klebsiella, pseudomonas, legionella
Aspiration pneumonia (7-14 days to become<unk> 
Bacteremia from nonpulmonary infection
Influenza leading to Bacterial superinfection (e.g. S. Aureus)
Pulmonary infarction
Infection as a result of penetrating chest trauma
Primary and metastatic neoplasms
Granulomatosis with polyangiitis (Wegener's), sarcoidosis
Lung abscess
Cavitary lesion with air-fluid level
Mediastinitis
Infected bullae
Pleuritis
Pleural fluid collection with bronchopleural fistula
Spontaneous pneumothorax
Loop of bowel extending through diaphragmatic hernia