The Collaborative International Dictionary of English v.0.48:

Pneumococcus \Pneu`mo*coc"cus\, n. [See Pneumo-, and
   Coccus.] (Biol.)
   A form of micrococcus found in the sputum (and elsewhere) of
   persons suffering with pneumonia, and thought to be the cause
   of this disease.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pneumococcus
    n 1: bacterium causing pneumonia in mice and humans [syn:
         pneumococcus, Diplococcus pneumoniae]