The word pneumonia means inflammation of the lung. Although this could result from noninfectious sources (e.g., a chemical inflammation secondary to aspiration), the great majority of cases are due to bacterial or nonbacterial infectious agents. Almost any bacterium, many species of fungi, and many viruses could be associated with pneumonia under some circumstances. Except for the newborn, in the early pediatric age group viruses are the most common etiology, followed by Staphylococcus infections. In older children and young adults, viruses still markedly predominate, but pneumonococci become more prevalent. In middle-aged and older adults, pneumococci are the most frequent bacteria and H. influenzae is also important, although viruses still are more frequent numerically. Mycoplasma pneumoniae is a very important cause of pneumonia in older children, young adults, and middle-aged adults but may appear at any age. In debilitated persons, alcoholics, persons with depressed immunologic defenses, and the elderly, pneumococci are still very important, but other bacteria become much more common, especially Staphylococcus and Klebsiella. Staphylococcal pneumonia is particularly likely to occur following a viral pneumonia, such as influenza. Legionella infections are assuming more importance in adults and the elderly, although their true incidence is not known because Legionella is not tested for routinely.

The most important nonbacterial agents producing lung infection are the respiratory-syncytial virus, influenza virus, the Aspergillus fungus, and an organism classified as a parasite, Pneumocystis carinii. Diseases caused by viruses, fungi, and parasites are discussed in other chapters.