Herpes - zoster is caused by :

“Herpes zoster is reactivation of latent chicken pox virus (varicella zoster virus) from the dorsal root ganglia.” Characteristic features of Herpes zoster Unilateral vesicular eruption within a dermatome often associated with severe pain. The dermatome from T3 to L3 are most frequently involved. Ramsay hunt syndrome : Rare form of herpes zoster affecting the facial nerve with eruption on tympanic membrane and external auditary canal with ipsilateral facial palsy. Most debiliating complication of herpes zoster in both the normal and immunocompromised host is pain