One of the oldest diseases in the world (Ramesses V of Egypt, who died in 1157 B.C.E. apparently had it), smallpox killed 300 million people worldwide in the 20th century [source: Flight]. The condition got its name to distinguish it the "great pox" (aka as syphilis). This contagious disease is spread by face-to-face contact with an infected person. Symptoms include fever, headache and severe back pain followed by telltale red pustules all over the body, which leave pitted scars [source: WHO].
Smallpox was also the first disease for which a vaccination became available. Britain's Dr. Edward Jenner had heard that milkmaids who had contracted the mild disease of cowpox never developed smallpox. In 1796, he tested the theory by injecting a boy with some pus from a cowpox pustule and saw that it gave him immunity to smallpox [source: Flight]. The breakthrough was enormous and paved the way for the science of vaccination. In 1959, the World Health Organization decided to eradicate the disease from the planet by isolating smallpox patients and vaccinating everyone in an area where smallpox was detected. In 1980, the organization declared victory; smallpox was no more [source: WHO].