Infection and disease are two different things. Infection means that the bacteria entered your body. You might get disease from it. Or your immunity might fight off and eliminate the bacteria. If you get the disease you get symptoms like fever cough etc etc. An infection without a disease doesn't come to notice. The bacteria enters the body and might get cleared off. However it leaves behind a strong specific immune response in the body as a tell tale sign of that infection. In quantiferon TB gold we measure levels of interferons which suggest the existing immunity against TB. It simply suggests a previous exposure to the bacteria and will be positive in infection or disease both. No need to worry.