The pneumococcus is by far the most common cause of bacterial pneumonia, particularly in the elderly. Pneumococci are carried in the upper respiratory tract of a large number of healthy individuals whose lungs are protected by the mucociliary escalator. In the elderly or in other individuals whose ciliary activity or respiratory drainage is likely to be impaired the organism can reach its pathogenic site in the lung and cause serious disease. Those most susceptible would be the bedridden, heavy smokers, alcoholics, or individuals who had suffered a recent respiratory viral infection or toxic damage to the mucociliary escalator.
Gold miners are highly susceptible because of toxic damage to the ciliary tree.
The capsule of the pneumococci is its most potent, perhaps its only, virulence factor. The capsule is antiphagocytic and non-encapsulated organisms are avirulent. The antiphagocytic properties of the capsule allow the pneumococci to grow in the lung. This results in an inflammatory response in which fluid accumulates in the lung, preventing its normal function. Anticapsular antibody is protective and curative.