A 33-year-old male with AIDS and a history of shingles develops a severe, multifocal encephalitis. Therapy is instituted with acyclovir, but the man dies on the fouh day of his hospital admission. Which of the following viruses is the most likely cause of his encephalitis?

The specific clue to the cause of the severe encephalitis in this AIDS patient is the history of shingles, due to reactivation of the herpes zoster-varicella virus. In otherwise healthy adults, the virus (which is usually introduced to the body as a childhood case of chickenpox) remains dormant in a dorsal root ganglion, only to reactivate in later life, causing a painful vesicular eruption that characteristically conforms to the distribution of a single dermatome. In AIDS patients, the virus can cause a severe, multifocal encephalitis that may be resistant to acyclovir therapy. Cytomegalovirus can cause disseminated disease (including brain infection) in AIDS patients, but is less likely in this patient, given the past history of shingles. Herpes simplex type I and herpes simplex type II can cause disseminated disease (including brain infection) in AIDS patients, but is less likely in this patient, given the past history of shingles, and the lack of response to acyclovir.