Varicella

Varicella, more commonly known as chickenpox, is a very contagious viral disease. It usually affects children and causes numerous small sores approximating a rash over the entire body. It will resolve itself within 10–14 days without treatment and is rarely fatal. However, the pustules are very uncomfortable, causing intense itching which should be treated to avoid secondary infections from broken sores.
The disease is somewhat more serious in persons who have a suppressed immune system, pregnant women (where it can cause birth defects), and adults who did not have the disease as a child. In these patients, antiviral medications are advised to prevent complications. However, they're not recommended in normal childhood cases.
Shingles is a complication of a second infection of varicella in older persons who have the disease. It usually causes fever, fatigue, and minor skin irritation for about 7–10 days before resolving itself.
There is now a vaccine for chickenpox, the last of the so-called childhood diseases to have one developed.
Chickenpox at NIH
Chickenpox at Wikipedia
Chickenpox at Mayo Clinic
Varicella