EPIDEMIOLOGY OF TUBERCULOSIS—PARR 481 
ter. The sensitized individual possesses a new reaction pattern, which 
he will keep as long as viable tubercle bacilli remain in his body. 
Fortunately, the great majority of sensitized individuals do not 
progress farther toward clinical tuberculosis. Such individuals are 
harmless to others in their environment, for the tubercle bacilli caus- 
ing the sensitization are locked within their bodies. Indeed, as Long 
has so well pointed out, the tuberculous individual does not enter into 
the epidemiological picture until his pathology is well advanced. 
Large lesions caseate, liquefy, and erode into bronchi where bacilli 
are spread farther within the lung of the hapless patient or expecto- 
rated to the outside world. Interestingly enough, the number of 
tubercle bacilli becomes very great in an area of just this type, whereas 
they might have been rather few in the same area a month earlier. 
Invasiveness 
Aggressive weapons of the 
microbe 
Virulence 
Correlation of Avenue of 
Approach with Tropism 
Frequenoy of Attack 
Magnitude of Assault 
Rapidity.of Progress of Astio 
v L 
Resistant Susceptible 
Ficure 2.—Some of the factors entering into the host-parasite relationship, which 
have much to do in determining the outcome of an infection. 
Only a few of those who become tuberculin positive for the first time 
will progress to the point where roentgenological evidence can be 
obtained that they are ill, and of these by no means all will advance 
farther to the point where clinical symptoms can be noted. Further- 
more, if taken at the stage of minimal tuberculosis, the disease is easy 
to arrest. Even if arrested the individual will still, for a long time, 
likely for life, harbor some of the tubercle bacilli that multiplied 
within his body. It may seem odd that one can be in good health and 
play host to pathogenic organisms. Such a healthy arrested case 
should not be a source of danger to others, but it is important to point 
out that every extensive survey of adults reveals some of these indi- 
viduals who are not satisfactorily arrested cases and who continue to 
work or even attempt to enlist in the Army or Navy while really suf- 
