A patient is suffering from painful vesicular eruption at T-4 dermatome. The cause is:
(Refer: Fitz Patrick’s Dermatology in General Medicine, 7th edition, pg no: 2383-2393)
Herpes zoster re-activation

Varicella zoster virus (VZV) is the etiology of varicella (chickenpox) and herpes zoster (shingles).
Herpes zoster represents reactivation of latent varicella infection and develops in approximately 20% of healthy adults and 50% of immunocompromised persons.
IP ranges from 11 to 20 days
Varicella is extremely contagious, with 80-90% of susceptible household contacts developing clinical infection
Herpes zoster classically is a disease of adulthood that affects those older than 50 years of age more severely, it is common in younger persons who had primary varicella infection within the first year of life.
Herpes zoster appears upon reactivation of VZV, which may occur spontaneously or may be induced by stress, fever, radiation therapy, tissue damage (i.e., trauma) or immunosuppression.
During a herpes zoster infection, the virus continues to replicate in the affected dorsal root ganglion and produces a painful ganglionitis.
Inflammation and neuronal necrosis can result in a severe neuralgia that intensifies as the virus spreads down the sensory nerve.

Diagnosis and Pathology

Clinical history and the physical examination
Tzanck smear and/or a DFA (direct fluorescent antibody)
Viral culture, Serology and PCR

Treatment
EBV infection
Herpes zoster
CMV infection
Herpes simplex