Chicken pox is an infectious disease caused by the varicella zoster virus. It causes an itchy rash that spreads over the body and develops into fluid-filled blisters that eventually crust over. While usually mild and self-limiting in children, it can cause serious complications in adults and those with weak immune systems. Vaccination has significantly reduced cases in the US since being introduced in 1995. Treatment focuses on relieving itching and symptoms, with antiviral drugs used in severe cases.