Smallpox is the most lethal disease in history, killing an estimated 30 crore people in the 20th century alone.
Dr Henderson loaned from CDC lead the team from WHO in an effort to eradicate smallpox and he succeeded. He was also an excellent manager co-ordinating between various organizations and also Soviet Union(during the height of cold war). More on his heroics can be read here. Here is his wikipedia page.
Reading about him puts things in perspective when tech CEOs proclaim “We are changing the world” or anyone for that matter. Here is an XKCD comic that drives the point home.