CONDITIONS MODIFYING INFECTION. 105 



infection with the streptococcus. The virulence of the bacil- 

 lus of malignant oedema is increased when it is associated 

 with the Bacillus prodigiosus, a non-pathogenic organism. 

 Typhoid cultures are increased in virulence when associated 

 with dead cultures of the Bacillus coli communis. 



These mixed infections play a very important role clini- 

 cally, as the course of the disease is influenced considerably 

 thereby. A mixed infection which contains the streptococcus 

 is a very intractable process, and much more serious than a 

 simple infection. Occasionally one organism will inhibit the 

 growth of another, and thus diminish its virulence. A mixed 

 infection may convert a local into a general infection. Epi- 

 demics are in a measure due to increased virulence of the 

 germ. The virulence is exalted by the rapid passage of the 

 organism from one individual to another. 



Number : A small number of virulent germs may pro- 

 duce disease as rapidly as a large number would ; whereas a 

 large number is necessary if the germs are attenuated. 

 Infection with a small number of any organism possessed of 

 little virulence usually does not result in disease. A number 

 that would be pathogenic for a mouse would not be patho- 

 genic for an elephant. With these modifications, the number 

 of the invading bacteria is of importance, otherwise not. 



AVENUE OF INFECTION : Bacteria, in order to produce 

 their characteristic results after invasion of the body, must 

 have entered the body through the proper channel. The 

 avenue of entrance will modify the infection. The typhoid 

 bacillus when injected into the subcutaneous tissues does 

 not produce typhoid fever, but simply a local abscess. When 

 the tubercle bacillus enters through the respiratory or intes- 

 tinal tract, it produces typical tuberculosis ; but when infec- 

 tion occurs through the skin, a local tuberculosis results 

 (lupus), which runs an extremely slow course and does 

 not tend to end fatally. The streptococcus injected sub- 

 cutaneously produces erysipelas or extensive suppuration. 

 When injected directly into the blood-current or lymph- 

 channels, it produces septicaemia or " blood-poisoning ". Inha- 

 lation of the pneumococcus is followed by lobar pneumonia ; 

 when it enters by any other route, it causes suppuration and 



