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Hospital-acquired (nosocomial) infections are those developing ≥48 hours after hospital admission or within 90 days of recent hospitalization, surgery, or mechanical ventilation.

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The most common radiologically detectable form is hospital-acquired pneumonia (HAP), often overlapping with ventilator-associated pneumonia (VAP) and health-care–associated pneumonia (HCAP).

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Etiopathology

Microbiology:

| Predominant organisms: | • Staphylococcus aureus (esp. MRSA) • Gram-negative bacilli: Pseudomonas aeruginosa, Klebsiella pneumoniae, E. coli, Proteus, Acinetobacter. | | --- | --- | | Aspiration and prolonged ventilation. | Anaerobic flora | | Immunocompromised | • Fungal agents (Candida, Aspergillus) • Opportunistic pathogens (Nocardia, CMV, Pneumocystis) |

Pathophysiology

Micro-aspiration of colonized oropharyngeal secretions around endotracheal tubes or through nasogastric devices.
Ventilated patients develop biofilm-associated infections of tracheal tubes and dependent lung segments.
HAP can progress to necrotizing pneumonia, abscess, or empyema if untreated.

Radiology

Pattern	Probable Etiology	Typical CT Findings
Lobar consolidation	Bacterial (HAP / CAP)	Homogeneous segmental opacity, air bronchograms
Patchy peribronchial opacities	Bronchopneumonia / VAP	Multifocal ground glass ± centrilobular nodules
Cavitary necrosis	Staph, Klebsiella, Pseudomonas	Thick-walled cavities, air-fluid levels
Diffuse GGO + crazy paving	Pneumocystis, viral, Legionella	Bilateral ground glass with septal thickening
Peripheral rounded nodules	Septic emboli	Peripheral cavitating nodules with feeding-vessel sign

Complications

Pulmonary abscess: Air-fluid level within parenchymal cavity.
Empyema: Pleural collection with enhancing “split pleura” sign.
Septic emboli: Peripheral cavitary nodules from bacteremia or endocarditis.
Bronchopleural fistula: Persistent air leak from abscess or empyema rupture.