Both chickenpox and shingles are caused by the varicella-zoster virus (VZV). Chickenpox occurs when a person catches the virus for the first time, resulting in fever and blisters that are typically over the whole body. Following the first chickenpox infection, the virus remains dormant in the body for life. Shingles is an episode of the same virus reactivating, typically causing a cluster of skin blisters that can result in chronic pain lasting weeks to months. Shingles can also affect the eyes or result in other complications, especially in persons with weakened immune systems. An individual can have shingles more than once.

Non-immune persons (persons who have never had chickenpox and have never received the chickenpox vaccine) could be infected with VZV from someone who has shingles and develop chickenpox. However, persons with shingles are less likely to spread VZV than those with chickenpox.

Among adults, older adults and immunocompromised individuals are more vulnerable to chickenpox infection and shingles, and severe disease complications.