**Core Concept**
Chickenpox is a highly contagious illness caused by the reactivation of the varicella-zoster virus (VZV), a member of the herpesvirus family. This virus primarily affects children and young adults, leading to a characteristic rash and fever.

**Why the Correct Answer is Right**
The varicella-zoster virus is a neurotropic virus that initially infects the respiratory tract and then spreads to the central nervous system, where it remains dormant for years. Upon reactivation, it travels to the skin, causing the characteristic rash and fever associated with chickenpox. The virus infects the cells of the epidermis and dermis, leading to the formation of vesicles and pustules.

**Why Each Wrong Option is Incorrect**
**Option A:** Herpes simplex virus (HSV) is incorrect because, while it is also a member of the herpesvirus family, it primarily causes oral and genital herpes, not chickenpox. HSV primarily infects mucosal surfaces and skin, but it does not cause the characteristic rash of chickenpox.

**Option B:** Rubella virus is incorrect because it causes German measles, a distinct illness with a different rash and clinical presentation. Rubella is also a viral illness, but it is not associated with the reactivation of a latent virus like chickenpox.

**Option C:** Coxsackievirus is incorrect because it is a type of enterovirus that causes a range of illnesses, including hand-foot-and-mouth disease. Coxsackievirus is not associated with the characteristic rash of chickenpox.

**Clinical Pearl / High-Yield Fact**
The varicella-zoster virus can remain dormant in the dorsal root ganglia for years, leading to the development of shingles (herpes zoster) in older adults. This is an important consideration for clinicians, as shingles can be a painful and debilitating condition.

**Correct Answer:** D. Varicella-zoster virus.