One test to determine whether a person has been infected with tuberculosis consists of injecting the person with proteins extracted from the tuberculosis bacterium  Once a person has been infected by a bacterium, the person' s immune system subsequently recognizes certain proteins present in that bacterium and attacks the bacterium  This recognition also takes place in the test and results in a skin irritation at the injection site
Hence the physicians who designed the test reasoned that anyone who reacts in this manner to an injection with the tuberculosis proteins has been infected with tuberculosis
The ability of the proteins present in the tuberculosis bacterium to trigger the skin irritation is exclusive to that bacterium.