IMMUNITY IN TUBERCULOSIS—FLEXNER. 639 
and its capacity to confer immunity. Unless the bacillus has the 
power to gain some foothold in the body it affords no protection; if 
on account of high pathogenic power or virulence it easily gains a 
foothold, then it brings about infection. To choose a culture of tu- 
bercle bacilli of just the right grade of virulence is one of the condi- 
tions, apparently, of successful experiment, as it must also be, in view 
of this fact, one of the difficulties of the method. The same difficulty 
has been encountered in the practical carrying out of this method of 
immunization in cattle. Several series of guinea pigs were inoculated 
with tubercle bacilli as follows: (a@) With dead bacilli, (4) with living 
bacilli from cold-blooded animals, (c) with a culture of human bacilli 
cultivated artificially for more than twenty years which produces on 
inoculation no appreciable local lesions and never tends to generalize, 
and (d) another human culture cultivated artificially for more than 
fourteen years, which still causes in all the pigs slightly enlarged 
inguinal glands near the site of inoculation, and occasionally brings 
about slight caseation of the nodes with a tendency to partial general- 
ization of the virus. The dead bacilli and the bacilli from cold- 
blooded species gave no protection; the second human culture, by 
reason of its greater invasive properties, protects better than the first, 
which is almost devoid of power to grow in the animal body. In no 
case, however, was the growth of the virulent bacilli wholly sup- 
pressed. 
In man the question of acquired immunity has been answered by 
many authorities, as far as the main considerations go, in the negative. 
A large number of well-observed facts demonstrates that a person who 
has suffered from localized tuberculosis of the lymph glands—scrofula 
so-called—or other form of local tuberculosis, can not count upon an 
immunity from pulmonary tuberculosis. And yet it can, I think, be 
shown by reference to statistics that in man there exists a refractory 
condition which becomes increased after infection, since the number 
of persons who have been the victims, at some period of their life, of 
a tuberculous infection, is very large in comparison with the number 
who die of this disease, or the even larger number who develop severe 
forms of it. Hirsch gives the mortality of tuberculosis as compared 
with deaths from all other causes as 3: 22; in other words, tuberculosis 
claims as victims of death one in every seven persons. This propor- 
tion does not, however, express the morbidity from tuberculosis, 
which is, in reality, far greater than these figures indicate. It is 
difficult to secure by vital statistics reliable data of the incidence of 
tuberculosis; but trustworthy observations made at autopsies upon 
human beings indicate that as many as 90 per cent of persons, dying 
from all causes, have at some period of their life been the victims of a 
tubercular infection. In far the greater number of instances the 
disease remains fixed in the bronchial or other lymphatic glands or 
