Zoster or shingles, a dermatomal-vesicular infection, is caused by reactivation of latent varicella–zoster virus (VZV). Primary infection with VZV causes varicella. Zoster occurs following waning VZV cellular immunity and presents as a painful rash involving one or two adjacent dermatomes.

The rash is preceded by prodromal pain over the affected dermatome. It may be accompanied with headache, photophobia, malaise with fever being less common. Prodromal pain lasts three to four days but it can extend to a week. A maculopapular rash develops into clusters of vesicles. Characteristic rash appears as vesicles on an erythematous base.  New vesicles continue to appear over three to four days. The rash is usually accompanied by the same pain during prodrome but this acute phase pain can worsen, improve or appear for the first time. VZV can cause viremia. In immunocompromised persons, extensive viremia can lead to disseminated form of zoster.

Postherpetic neuralgia (PHN) is the most common complication of herpes zoster. It is diagnosed as pain persisting for an extended period after onset of zoster rash. Other complications include ophthalmic involvement, bacterial superinfections of the lesions, cranial and peripheral nerve palsies, meningoencephalitis, pneumonitis, hepatitis and acute retinal necrosis.