Klebsiella pneumonia is caused by Klebsiella pneumoniae, an enteric gram-negative rod. Klebsiella pneumoniae usually enters the lung via aspiration of oral secretions and can result in both community-acquired pneumonia as well as hospital-acquired pneumonia. Most cases occur in the hospital.

Middle-aged or older men are affected most frequently, and alcohol use disorder is the most common risk factor. Other patients at risk include those with diabetes or a malignancy, the elderly in nursing homes, debilitated patients receiving antibiotics, and those with chronic cardiopulmonary disease and renal disease.

The onset of symptoms is sudden with pleuritic chest pain, chills, prostration, moderate fever, dyspnea, and productive cough. The sputum can be thick and bloody, greenish, and also have a "currant jelly" appearance. The white blood cell count is usually elevated, but neutropenia appears to be frequent in those with cirrhosis and Klebsiella infection. Bacteremia can occur in up to 25% of cases. Complications can include lung abscesses, pericarditis, empyema, and meningitis. Rarely, there can be eye complications. In hypervirulent cases, pyogenic abscesses can form in the liver, central nervous system, urinary tract, soft tissues, and other sites.