Chickenpox, also known as varicella, is a highly contagious disease caused by the initial infection with varicella zoster virus (VZV). The disease results in a characteristic skin rash that forms small, itchy blisters, which eventually scab over. It usually starts on the face, chest, and back and then spreads to the rest of the body. Other symptoms may include fever, feeling tired, and headaches.
When SG-1 arrived on the Medieval planet, they discovered a young girl named Mary who was revealed to be suffering from chickenpox. (SG1: "Demons")