Lung abscess is a type of liquefactive necrosis of the lung tissue and formation of cavities (more than 2 cm) containing necrotic debris or fluid caused by microbial infection. This pus-filled cavity is often caused by aspiration, which may occur during anesthesia, sedation, or unconsciousness from injury. Alcoholism is the most common condition predisposing to lung abscesses. Lung abscess is considered primary (60%) when it results from existing lung parenchymal process and is termed secondary when it complicates another process e.g. vascular emboli or follows rupture of extrapulmonary abscess into lung. Signs and symptoms Onset of symptoms is often gradual, but in necrotizing staphylococcal or gram-negative bacillary pneumonias patients can be acutely ill. Cough, fever with shivering, and night sweats are often present. Cough can be productive of foul smelling purulent mucus (≈70%) or less frequently with blood in one third of cases). Affected individuals may also complain of chest pain, shortness of breath, lethargy and other features of chronic illness. Those with a lung abscess are generally cachectic at presentation. Finger clubbing is present in one third of patients. Dental decay is common especially in alcoholics and children. On examination of the chest there will be features of consolidation such as localized dullness on percussion and bronchial breath sounds. Complications Although rare in modern times, can include spread of infection to other lung segments, bronchiectasis, empyema, and bacteremia with metastatic infection such as brain abscess. Other complications from under recognition, undertreatment, and untreated underlying causes include rupture into pleural space, pleural fibrosis, trapped lung, respiratory failure, bronchopleural fistula, and pleurocutaneous fistula. Causes Conditions contributing to lung abscess Aspiration of oropharyngeal or gastric secretion Septic emboli Necrotizing pneumonia Vasculitis: Granulomatosis with polyangiitis Necrotizing tumors: 8% to 18% are due to neoplasms across all age groups, higher in older people; primary squamous carcinoma of the lung is the most common. Organisms In the post-antibiotic era pattern of frequency is changing. In older studies anaerobes were found in up to 90% cases but they are much less frequent now. Anaerobic bacteria: Actinomyces, Peptostreptococcus, Bacteroides, Fusobacterium, Parvimonas micra species,Microaerophilic streptococcus: Streptococcus milleri Aerobic bacteria: Staphylococcus, Klebsiella, Haemophilus, Pseudomonas, Nocardia, Escherichia coli, Streptococcus, Mycobacterium Fungi: Candida, Aspergillus, Histoplasma, Blastomyces, and Coccidioides Parasites: Entamoeba histolytica, Paragonimus Diagnosis Imaging studies Lung abscesses are often on one side and single involving posterior segments of the upper lobes and the apical segments of the lower lobes as these