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first three. These diseases therefore afford particularly fa- 

 vourable conditions for the development of Pfeiffer's bacillus. 

 Its growth is also favoured but to a less extent by infections 

 which involve a more limited area of mucous membrane, — 

 simple angina, diphtheria, scarlatina, and by catarrhal con- 

 ditions or the disposition to such, induced by non-bacterial 

 causes. The last case will explain its occurrence as the only 

 bacterium in bronchitis and similar complaints. The reason 

 Pfeiffer's bacillus occurs most constantly in whooping-cough 

 and measles, is on account of the great distribution and ende- 

 mic character of these diseases, so that the soil for its nourish- 

 ment is always present. When once the bacillus has spread 

 itself to its utmost extent in this favourable ,,soil" nothing will 

 force it to relinquish its foothold. The conditions are other- 

 wise in such a pronounced epidemic disease as influenza. Consi- 

 der an epidemic in its very early stages. It spreads over a 

 population the large majority of which do not harbour Pfeif- 

 fer's bacillus. At first it only exceptionally occurs in the pa- 

 tients, but in those patients in whom it is present it will mul- 

 tiply rapidly and spread from them, first of all in the same 

 congenial soil, which may happen by transference together with 

 the specific virus of influenza to healthy persons who thus 

 become infected with influenza as well as Pfeiffer's bacillus, 

 and also by transference to other influenza patients who then 

 get an additional infection with Pfeiffer's bacillus. Furthermore 

 it can be transferred to healthy persons without their con- 

 tracting influenza. The frequency with which this takes place 

 seems to vary greatly at different times and places even though 

 its distribution in influenza is the same. This can be ex- 

 plained by assuming that before the appearance of the influenza 

 it lacked the capacity of developing in healthy persons. This 

 power w r as first developed in its new conditions of life in 

 influenza patients and it is likely to take some time before it 

 has acquired the characters which are essential to its flou- 

 rishing in healthy persons. Both from originally healthy people 

 and from those who retain their Pfeiffer's bacilli after the in- 

 fluenza has finally disappeared, Pfeiffer's bacillus may be trans- 

 ferred to other healthy persons and to influenza patients. In 

 this manner the bacillus gradually spreads until sooner or 

 later it is present in almost every influenza patient and in a 



