32 INFECTION 



effects on experimental animals and specify what attenuation is 

 meant when they are to be used as vaccine. A very interesting 

 virulent, yet attenuated, form of streptococcus is to be met in sub- 

 acute endocarditis. These organisms produce serious or even fatal 

 valvulitis, and yet have no effect upon other organs or upon lower 

 animals. They are extremely hard to remove from the body. 

 They have accustomed themselves to residence in the body, have 

 established a balance or 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 

 opposition of 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 meta- 

 bolic products of foreign bacteria. Cultures of pneumococci may 

 be made so virulent by the first means that only one pneumococcus 

 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 streptococcic infection 

 added to virulent diphtheria infection, greatly increases the severity 

 of the disease. 



The secondary streptococcic infection in small-pox and in 

 phthisis complicates the primary infection and frequently causes 

 death of the individual affected. The hectic fever and sweats of 

 phthisis are due to this secondary infection. Combinations of 

 diphtheria bacilli and pneumococci increases the virulence of the 

 latter. The transference of infection 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 individ- 



