Polio, measles, chicken pox—these now-rare diseases were once common in the U.S. But they haven't been eliminated, as anyone who has seen the outbreaks triggered by the anti-vaccination movement can tell you. In fact, only one disease has ever been completely eliminated by humans: smallpox.
In the TED Ed video above, we learn exactly how a disease that killed 300 million people worldwide in the 20th century is now . . . gone. The short answer to how this happened: Smallpox is only spread through human hosts, so vaccinating all of an infected person's potential contacts is incredible effective. That worked quickly in most industrialized countries, but getting rid of smallpox in the developing world took decades.
In India, a smallpox hotspot, health workers visited every household in the entire country looking for cases. By 1980, smallpox was finished.
This same methodology wouldn't be as effective for diseases like Ebola, which survive in animal carriers as well as in humans. The smallpox rash was also a giveaway that a person was infected; other viruses aren't so apparent. Check out the video, and this TED Ed lesson, for more.