**Core Concept**
Varicella-zoster virus (VZV) causes chickenpox and reactivates as herpes zoster (shingles) in adults. The virus remains latent in the dorsal root ganglia. Reactivation occurs due to reactivation of latent VZV, not due to re-infection.

**Why the Correct Answer is Right**
Reactivation of VZV from the latent state is the primary cause of herpes zoster. This is facilitated by cellular factors such as aging, immunosuppression, and stress, which impair the host's ability to suppress viral replication. The viral genome remains intact in the latent state, and reactivation is not due to re-infection with a new strain of the virus.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as reactivation of latent VZV is the primary cause of herpes zoster, not re-infection with a new strain of the virus.
**Option B:** Incorrect, as the primary cause of herpes zoster is reactivation of latent VZV, not a different viral infection.

**Clinical Pearl / High-Yield Fact**
It is essential to note that herpes zoster is a reactivation of latent VZV, not a new infection. This has implications for vaccination strategies, as the shingles vaccine aims to prevent reactivation of latent VZV, not to provide immunity against new infections.

**Correct Answer:** D.