288 BACTERIOLOGY. 



streptococcus, pneumococcus, and influenza bacillus. 

 Besides these many other varieties are met with occa- 

 sionally in individual cases. What the influence of 

 this secondary or mixed infection is, under all circum- 

 stances, is not exactly known; but generally the effect 

 is an unfavorable one, and not infrequently on their 

 invasion the disease takes on a septicsemic character. 

 For the technique employed in examining sputa for 

 mixed infection, see later in this chapter. 



Immunization. As in other infectious diseases, many 

 attempts have been made to produce an artificial immu- 

 nity against tuberculosis, but so far the results have 

 been unsatisfactory. Among the numerous medicinal 

 agents that have been tried to protect animals against 

 the action of the tubercle bacillus may be mentioned 

 tannin, menthol, sulphuretted hydrogen, mercuric chlo- 

 ride, creosote, creolin, phenol, arsenic, eucalyptol, etc. 

 Various inoculation experiments with cultures of the 

 tubercle bacilli and their products have been made, 

 and though the results reported in some cases have 

 been temporarily favorable, immunization has never 

 been satisfactorily produced. 



Koch's Tuberculin. The discovery by Koch of toxins 

 in cultures of the tubercle bacillus which possess prop- 

 erties which explain its pathogenic power must rank 

 as one of the first importance in scientific medicine, 

 on account of what it has led up to, even if as ap 

 pears probable the final verdict may be that its thera- 

 peutic value in the treatment of tubercular diseases in 

 man is very slight. 



Tuberculin contains all the products of the growth 

 of the tubercle bacilli in the nutrient bouillon as well 

 as some substances extracted from the bodies of the 



