Smallpox (430 BC? - 1979):
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.
Spanish Flu (1918 - 1919):
Killed 50 to 100 million people worldwide in less than 2 years
In 1918 and 1919, the Spanish Flu pandemic killed more people than Hitler, nuclear weapons and all the terrorists of history combined. (A pandemic is an epidemic that breaks out on a global scale.) Spanish influenza was a more severe version of your typical flu, with the usual sore throat, headaches and fever. However, in many patients, the disease quickly progressed to something much worse than the sniffles. Extreme chills and fatigue were often accompanied by fluid in the lungs. One doctor treating the infected described a grim scene: "The faces wear a bluish cast; a cough brings up the blood-stained sputum. In the morning, the dead bodies are stacked about the morgue like cordwood."
If the flu passed the stage of being a minor inconvenience, the patient was usually doomed. There is no cure for the influenza virus, even today. All doctors could do was try to make the patients comfortable, which was a good trick since their lungs filled with fluid and they were wracked with unbearable coughing. The "bluish cast" of victims' faces eventually turned brown or purple and their feet turned black. The lucky ones simply drowned in their own lungs. The unlucky ones developed bacterial pneumonia as an agonizing secondary infection. Since antibiotics hadn't been invented yet, this too was essentially untreatable. The pandemic came and went like a flash. Between the speed of the outbreak and military censorship of the news during World War I, hardly anyone in the United States knew that a quarter of the nation's...