Hello This reference is from the standard test book so read it very carefully because your are in dilemma because sufferer is your very young sister. Herpes Zoster represents a reactivation of Varicella-zoster virus ( VZV) from dorsal root ganglia and usually manifests as a UNILATERAL vesicular eruption within a dermatome, often associated with severe pain. The usual duration of disease is 7-10 days , but it may take as long as 2-4 weeks for the skin to return normal. Alone in USA ~12 million cases each year , the incidence is highest among patients -60 years of age but young age is not exception ( so your sister may be the the victim in this age group and this is not related to HIV infection at all ). However in a patient of HIV infection herpes zoster may be but HERPES ZOSTER in any way don t indicate towards HIV infection. In my clinic I have seen so many young and old cases with HERPES zoster and simple antiviral treatment is perfect treatment . Post herpetic neuralgias is most common complication of this disease and it may persists even for years. Diagnosis ;Definitive diagnosis requires isoaltion of VZV in culture , detection of VZV by molecular means ( PCR immunofluorescent staining of cells from the lesions base). So please get in this test and be tension free. Hope this will help you. This answer is in detail so that you can understand in better way. Good luck.