Killed more than 300 million people worldwide in the 20th century alone, and most of the native inhabitants of the Americas
Smallpox (also known by the Latin names Variola or Variola vera) is a contagious disease unique to humans. Smallpox is caused by either of two virus variants named Variola major and Variola minor. The deadlier form, V. major, has a mortality rate of 30–35%, while V. minor causes a milder form of disease called alastrim and kills ~1% of its victims. Long-term side-effects for survivors include the characteristic skin scars. Occasional side effects include blindness due to corneal ulcerations and infertility in male survivors.
Smallpox killed an estimated 60 million Europeans, including five reigning European monarchs, in the 18th century alone. Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one third of the survivors became blind.
As for the Americas, after the first contacts with Europeans and Africans, some believe that the death of 90 to 95 percent of the native population of the New World was caused by Old World diseases. It is suspected that smallpox was the chief culprit and responsible for killing nearly all of the native inhabitants of the Americas. In Mexico, when the Aztecs rose up in rebellion against Cortés, outnumbered, the Spanish were forced to flee. In the fighting, a Spanish soldier carrying smallpox died. After the battle, the Aztecs contracted the virus from the invaders' bodies. When Cortes returned to the capital, smallpox had devastated the Aztec population. It killed most of the Aztec army, the emperor, and 25% of the overall population. Cortés then easily defeated the Aztecs and entered Tenochtitlán, where he found that smallpox had killed more Aztecs than had the cannons.
Smallpox was responsible for an estimated 300–500 million deaths in the 20th century. As recently as 1967, the World Health Organization (WHO) estimated that 15 million people contracted the disease and that two million died in that year. After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the eradication of smallpox in 1979. To this day, smallpox is the only human infectious disease to have been completely eradicated from nature.