Półpasiec oraz ospa wietrzna wywołane są tym samym wirusem typu - [[Herpes varicella-zoster]] zbliżonym do wirusa opryszczki.

The causative agent for herpes zoster is varicella zoster virus (VZV), a double-stranded DNA virus related to the Herpes simplex virus group. Most people are infected with this virus as children, and suffer from an episode of chickenpox. The immune system eventually eliminates the virus from most locations, but it remains dormant (or latent) in the ganglia adjacent to the spinal cord (called the dorsal root ganglion) or the ganglion semilunare (ganglion Gasseri) in the base of the skull.^[16] Repeated attacks of herpes zoster are rare,^[11] and it is extremely rare for patients to suffer more than three recurrences.^[16]

Herpes zoster occurs only in people who have been previously infected with VZV; although it can occur at any age, approximately half of the cases in the USA occur in those aged 50 years or older.^[17] The disease results from the virus reactivating in a single sensory ganglion.^[4] In contrast to Herpes simplex virus, the latency of VZV is poorly understood. The virus has not been recovered from human nerve cells by cell culture and the location and structure of the viral DNA is not known. Virus-specific proteins continue to be made by the infected cells during the latent period, so true latency, as opposed to a chronic low-level infection, has not been proven.^[2][18] Although VZV has been detected in autopsies of nervous tissue,^[19] there are no methods to find dormant virus in the ganglia in living people.

Unless the immune system is compromised, it suppresses reactivation of the virus and prevents herpes zoster. Why this suppression sometimes fails is poorly understood,^[6] but herpes zoster is more likely to occur in people whose immune system is impaired due to aging, immunosuppressive therapy, psychological stress, or other factors.^[20] Upon reactivation, the virus replicates in the nerve cells, and virions are shed from the cells and carried down the axons to the area of skin served by that ganglion. In the skin, the virus causes local inflammation and blisters. The short- and long-term pain caused by herpes zoster comes from the widespread growth of the virus in the infected nerves, which causes inflammation.^[21]

As with chickenpox and/or other forms of herpes, direct contact with an active rash can spread VZV to a person who has no immunity to the virus. This newly infected individual may then develop chickenpox, but will not immediately develop shingles. Until the rash has developed crusts, a person is extremely contagious. A person is also not infectious before blisters appear, or during postherpetic neuralgia (pain after the rash is gone).^[11]