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.
S. pneumoniae is also a frequent cause of meningitis in adults. The meningitis may arise as a complication of pneumonia or sinusitis, in which the bacteria reach the meninges via the bloodstream, or it may result from a head injury which permits organisms from the nasopharynx to enter the meninges. As with all organisms that can cause bacteremia, endocarditis is a possibility, especially for those with damaged heart valves. In children, the pneumococcus is a very common cause of otitis media. The pneumococci are untypable in the Lancefield classification which is based on the antigenicity of a carbohydrate which is soluble in dilute acid and called the C carbohydrate.