AVENUES OF INFECTION 35 



lished a balance of poise between their offenses and the bodily 

 defenses and practically cannot be rapidly dislodged. These are 

 called fixed or fast strains. Such strains may be seen under other 

 conditions such as the typhoid bacillus in the gall-bladder. These 

 fast strains usually are found at places remote from intimate con- 

 tact with the defenses of the body, the leucocytes and blood serum 

 as in the cases cited. 



The malignancy of bacteria may be heightened in various ways : 

 (i) By passing them repeatedly through the bodies of susceptible 

 animals; (2) by cultivation in culture media in collodion sacs 

 placed in the abdominal cavities of animals; (3) by injections 

 mixed with other injurious substances, such as lactic acid, and the 

 metabolic products of foreign bacteria. Cultures of pneumococci 

 may be made so virulent by the first means that only one pneu- 

 mococcus is capable of setting up a fatal septicaemia in a rabbit. 

 By injecting attenuated diphtheria bacilli with streptococci into a 

 rabbit, the virulence of the bacilli can be raised, as mixed infection 

 often adds to the virulence of an organism. Malignant strepto- 

 coccic infection added to virulent diphtheria infection, greatly 

 increases the severity of the disease. The transference of infec- 

 tive agents from one person to another during an epidemic 

 increases the virulent action of the organism by reason of the 

 rapid passage from individual to individual. 



Mixed infections are those in which more than one kind of 

 virus is active. It is of course possible that two kinds may 

 originate a disease, but it is usual for one germ to initiate a process 

 and another to be superimposed upon it, usually intensifying the 

 lesions. The active ulcerative inflammation in tuberculous lungs 

 is usually due to be secondary effect of streptococci. 



The secondary streptococcic infection in small-pox and in 

 phthisis complicates the primary infection and frequently causes 

 death of the individual affected. 



The avenue of infection and the tissues infected alter the type of 

 the disease exceedingly. Streptococci invading the tonsils cause 



