Color enhanced transmission electron micrograph of smallpox viruses using a negative stain technique. Smallpox was an infectious disease caused by either of two virus variants, Variola major and Variola minor. The last naturally occurring case of smallpox (Variola minor) was diagnosed in 1977. The disease killed an estimated 400,000 Europeans annually during the closing years of the 18th century, and was responsible for a third of all blindness. Of all those infected, 20-60%, and over 80% of infected children, died from the disease. Smallpox was responsible for an estimated 300-500 million deaths during the 20th century. After vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the global eradication of smallpox in 1980. Magnification: unknown.