It's one thing to be able to treat a disease or educate people about how to prevent it. But completely eradicating it off the face of the Earth? That's an extremely rare victory, but it's one humans have notched against the once-devastating smallpox virus.
Scientists believe smallpox got its start in northern Africa some 10,000 years ago, giving it plenty of time to wreak havoc on the human race. By the 18th century things were pretty bad: The disease killed 14 percent of the Europeans who contracted it, or some 400,000 annually [sources: Riedel, Whipps]. Even worse was the way it nearly wiped out the indigenous people of North and South America, who had no resistance to diseases brought over by European colonizers.
Then came a British doctor named Edward Jenner, who, like many people of the time, noticed that milkmaids rarely got smallpox. He figured it might be because they often came down with a similar disease called cowpox. So Jenner took a little bit of goop from a milkmaid's cowpox sore and gave it to a young boy whom he then tried to infect with smallpox (it was 1796 so no one called out his questionable ethics). The boy never got sick, and the vaccine was born. Thanks to the widespread use of the vaccine, the World Health Organization declared smallpox eradicated in 1980, saving as many as 5 million lives annually [sources: Whipps, UNICEF].