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 septicemic 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, ete. 
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 
