236 THE ACUTE PNEUMONIAS 



coccus occurring in the healthy naso-pharynx is usually of 

 Type IV., i.e., belongs to the group least pathogenic to man. 

 The more pathogenic types are found almost exclusively in the 

 mouths of convalescents and of contacts and in rooms where 

 pneumonia cases have been nursed. While these types usually 

 rapidly disappear from convalescents and contacts they may per- 

 sist long enough to justify the view that certain persons may act 

 as carriers of the disease. 



It is more difficult to explain why sometimes the pneumo- 

 coccus is associated with a spreading inflammation, as in croupous 

 pneumonia, whilst at other times it is localised to the catarrhal 

 patches in broncho-pneumonia. It is quite likely that in the 

 former condition the organism is possessed of a different order 

 of virulence, though of this we have no direct proof. We have, 

 however, a closely analogous fact in the case of erysipelas ; this 

 disease, we have stated reasons for believing, is produced by 

 a streptococcus which, when less virulent,' causes only local 

 inflammatory and suppurative conditions. 



Summary. — We may accordingly summarise the facts re- 

 garding the relation of Fraenkel's pneumococcus to the disease 

 by saying that it can be isolated from nearly all cases of acute 

 croupous pneumonia, and also from a considerable proportion 

 of other forms of pneumonia. When injected into the lungs of 

 moderately insusceptible animals it gives rise to pneumonia. 

 We are therefore justified in holding that it is the chief factor 

 in causing croupous pneumonia, and also plays ap important 

 part in other forms. 



Immunisation against the Pneumococcus. — Animals can be 

 immunised against the pneumococcus by inoculation with 

 virulent cultures killed by heating at 55° C, with cultures 

 which have become attenuated by growth on artificial media, or 

 with the naturally attenuated cocci which occur under various 

 circumstances. Sometimes one or two injections, at intervals 

 of several days, are sufficient for immunisation, but the immunity 

 has been observed to be usually of a fleeting character and may 

 not last more than a few weeks ; a process of intensive and rapid 

 immunisation is described below. The serum of such immunised 

 animals mixed in vitro with pneumococci neutralises the 

 action of these in susceptible animals, may also protect against 

 subsequent inoculation with carefully regulated doses of pneu- 

 mococci, and if injected within twenty-four hours after inocula- 

 tion, may prevent death. Such serum also possesses an 

 agglutinating action in low dilutions on the pneumococcus 

 originating the immunity. 



