'Shingles' (a painful, unilateral, dermatomal vesicular rash) is caused by reactivation of which virus?

Shingles (herpes zoster) results from reactivation of latent varicella-zoster virus (the same virus causing chickenpox/varicella in primary infection), which has remained dormant in dorsal root or cranial nerve ganglia, and reactivates along a single dermatome, often during times of immunosuppression or aging.