It's a viral illness. It used to be extremely common, but isn't anymore now that we vaccinate against it. Prior to the vaccine, 100-200 children per year died in the USA as a result of complications of chicken pox. That number is now much lower. Once you have chicken pox, you have it forever. The virus lies dormant near your spinal cord, and sometimes reactivates. When it reemerges, it's called shingles.
Caused by a virus. Usually only children get chicken pox unless they've been vaccinated. They develop extremely itchy blister-like red spots all over the body. It is usually mild and goes away on its own, but can cause complications in adults and older children. Oatmeal or cornstarch baths can help with the itching. Severe symptoms are more common in children who are immunosuppressed or on steroids.