Responsible for an estimated 300 million to 500 million deaths during the 20th century alone, smallpox has been one of humanity's most merciless scourges. The virus primarily attacks skin cells, which leads to characteristic bumps, or macules, that form all over the body.
The smallpox vaccine was the first successful vaccine to be developed. It was discovered by Edward Jenner in 1796, after he noticed that people who caught the more innocuous cowpox virus seemed to be immune to the smallpox virus. A worldwide eradication campaign in the 20th century eventually led to the destruction of the smallpox virus. To this day, it remains the only virus that afflicted humans that has been 100 percent eradicated. The last known case of the disease occurred in Somalia in 1977.