**Core Concept**
Leprosy, also known as Hansen's disease, is a chronic bacterial infection caused by *Mycobacterium leprae*. It primarily affects the skin, peripheral nerves, and mucous membranes, leading to disfigurement and disability.

**Why the Correct Answer is Right**
Leprosy is characterized by a strong cell-mediated immune response, which is often inadequate to control the bacterial infection. The disease has two main forms: tuberculoid and lepromatous, depending on the host's immune response. In tuberculoid leprosy, the immune system mounts a strong response, whereas in lepromatous leprosy, it is impaired, leading to widespread bacterial dissemination.

**Why Each Wrong Option is Incorrect**
*Option A:* Leprosy is caused by a viral infection. Incorrect because leprosy is caused by the bacterium *Mycobacterium leprae*.
*Option B:* Leprosy primarily affects the central nervous system. Incorrect because leprosy primarily affects the peripheral nerves.
*Option C:* Leprosy is highly contagious. Incorrect because leprosy is not highly contagious; it is primarily spread through respiratory droplets or close contact with an infected person.

**Clinical Pearl / High-Yield Fact**
Leprosy can be effectively treated with multidrug therapy (MDT), which includes rifampicin, clofazimine, and dapsone. Early diagnosis and treatment are crucial to prevent long-term disability and disfigurement.

**Correct Answer: B. Leprosy primarily affects the peripheral nerves.**