The world officially became a slightly safer place in October, when the World Health Organization declared that polio’s type 3 strain had been eradicated. This strain — joining type 2, which was eradicated in 2015 — no longer exists anywhere in the world, outside of highly secure laboratories. (Type 1 is the only strain still at large.) Thanks to the hard work of thousands of dedicated individuals, these two strains will no longer cause devastating paralysis or death.

While it was once just a dream, permanently ending diseases has been within our power since 1980, when smallpox was eradicated after an intense campaign. This victory has saved roughly 200 million people who would otherwise have succumbed to the disease since then.

But other attempts to rid the world of diseases have not gone as smoothly. Doctors have been working on ending polio for 31 years, initially hoping it would be completely gone by 2000. Now, due to difficulties tracking the disease, the target eradication date for the remaining type 1 strain is 2023. Another pathogen nearing eradication is a parasite known as Guinea worm, but again, problems have complicated that campaign, and others as well.

So what was it about smallpox that made it so much simpler to eradicate? What makes an organism eradicable in the first place?

Fundamentally, if we want to get rid of a pathogen, we must have a way of stopping its transmission. Halt the spread, and you can isolate those infected without anyone else getting sick. Do a thorough enough job, and there won’t be any new cases anywhere in the world — the disease is eradicated. Theoretically, this process can take many forms. The deployment of an effective vaccine robs a disease of future hosts. Eliminating a key vector takes out the means of infection. And for a bacterial pathogen, antibiotic treatments can target the disease itself. But theory doesn’t always translate to practice in the real world.

For a sense of what actually works, smallpox provides the perfect case study: It turns out to be almost ideally suited to eradication. First, it’s a virus that only affects people, not animals. Wipe it out in humans, and that’s it, you’re done. (We’re not actually sure why smallpox is so choosy, and we’re unlikely to find out anytime soon, since little research today involves the deadly pathogen — and even then, it focuses on treatments and vaccine research over fundamental biology.)

Second, the disease makes its presence clearly and unambiguously known. It produces a rash that’s easy to identify and distinct from rashes caused by other diseases. And infections are not asymptomatic: You can’t be infected and contagious but still appear healthy. (Again, it’s not clear why this is.) These traits make it easier to track new cases and quickly stop outbreaks.

Read the rest of Tara C. Smith’s article at Quanta Magazine