Illustration of erythema (redness of the skin, top left), smallpox (top right) and chickenpox (bottom). Smallpox is an often-fatal disease caused by the Variola virus. Prior to vaccination (first performed in 1796) it killed millions of people each year. Chickenpox (varicella) is a common infectious disease of childhood caused by the varicella-zoster virus. It causes headache and fever, followed by the eruption of a rash over most of the body. From Tout le corps Humain by Henri Bouquet (1929).