**Core Concept**
The BCG (Bacillus Calmette-Guérin) vaccine is a **vaccine** used to protect against a specific infectious disease. It is a live, attenuated vaccine made from a weakened form of **Mycobacterium bovis**. The primary use of BCG vaccine is to provide immunity against **tuberculosis (TB)**.

**Why the Correct Answer is Right**
The BCG vaccine is administered to individuals to stimulate the immune system to produce **cell-mediated immunity** against **Mycobacterium tuberculosis**, the causative agent of TB. This immunity helps to prevent the development of severe forms of TB, such as **miliary TB** and **TB meningitis**, especially in children.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while BCG can have some non-specific effects on the immune system, it is not primarily used for this purpose.
**Option B:** Incorrect as BCG is not used to treat TB, but rather to prevent it.
**Option C:** Incorrect because although BCG has some applications beyond TB prevention, this is not its primary use.

**Clinical Pearl / High-Yield Fact**
The BCG vaccine is most effective in preventing severe forms of TB in children, and its efficacy wanes with age. It is an essential component of **national immunization programs** in many countries with high TB prevalence.

**Correct Answer:** D. Tuberculosis