Shingles is an infection caused by the same virus that causes chickenpox (the varicella-zoster virus, which is a type of herpes virus). You cannot develop shingles unless you have had a previous infection of chickenpox (usually as a child). Shingles can occur in people with suppressed immune systems, which includes people with HIV and people over 60 years of age (especially those with diabetes, cancer, or other diseases that can suppress immunity).
Up to 95 percent of people in the United States have antibodies against the varicella-zoster virus and many develop chickenpox at some point in their lives, usually when they are school-aged children. Even though the pox lesions heal, the virus does not die—it continues to live quietly in nerve roots near the spinal cord. While the immune system cannot kill the virus completely, it can prevent the virus from becoming active again, usually for the rest of an infected person's life. However, if the immune system becomes suppressed, the virus can escape the nerve roots and become active. Instead of coming back as chickenpox (varicella), it comes back as shingles (zoster).
When shingles occurs, it only affects one side of the body, usually in the form of a belt-like streak along a single line of nerves. The most common site is the back, upper abdomen, or face. It can also affect the eyes and more rarely the inner ear. Shingles can be very painful, but it can be treated.
You cannot transmit shingles to someone who has had chickenpox in the past or has been vaccinated against the varicella-zoster virus. However, the rash that occurs with shingles can “shed” the varicella-zoster virus. Someone who has not had chickenpox or has not been vaccinated against this virus can develop chickenpox if they come into contact with a shingles rash.
Approximately 3% to 5% of people infected with the varicella-zoster virus will experience shingles at some point in their lives, most of them after the age of 50. Shingles is 15 to 25 times more likely to occur in HIV-positive people, regardless of the CD4 cell count. In other words, the CD4 cell count doesn't need to be low for shingles to be a risk; it can develop even when the immune system appears relatively healthy. In HIV-positive people with significant immune suppression (CD4 cell count below 50), there is an increased risk of zoster infection of other parts of the body, including the retina at the back of the eye. This can result in rapid blindness.