Polio just got one step closer to becoming the second human disease to be fully wiped out. On Thursday, World Polio Day, the World Health Organization announced that type 3 poliovirus has been eradicated worldwide.

For polio to be fully eradicated, all three wild polio strains — types 1, 2, and 3 — need to stop circulating. The three strains all cause the same horrible symptoms, including paralysis and death, but are virologically distinct.

Type 2 was eradicated back in 2015; the last case of type 3 polio surfaced in northern Nigeria in 2012 and the virus hasn’t been seen since. A poliovirus can be considered eradicated if it hasn’t been detected for three years.

“[The eradication of type 3 polio] is a significant achievement that should reinvigorate the eradication process and provides motivation for the final step — the eradication of wild poliovirus type 1,” said David Salisbury, chair of the independent Global Commission for the Certification of Poliomyelitis Eradication, in a statement Thursday.

Today, only type 1 remains at large — in Afghanistan and Pakistan. If it’s eradicated, polio will join smallpox as the only two human scourges wiped off the face of the planet. (A third disease that’s been eradicated, rinderpest, is spread mainly in cattle.) And today’s news is a step in that right direction.

But getting from the 94 cases of type 1 polio reported this year to zero won’t be easy. And, ironically, one of the biggest polio obstacles that remains was actually brought on by the immunization effort: an outdated version of the vaccine that can spread the virus.

The push to eradicate polio has been a huge global health success

Polio mainly affects children under the age of 5. At its peak in the middle of the 20th century, the disease killed half a million people every year. In 1988, when WHO launched the eradication program, there were more than 350,000 cases in 125 polio-endemic countries. Since then, cases of wild poliovirus have decreased by over 99 percent, according to WHO, to just 94 this year.

In the two countries where the type 1 virus is still endemic, Pakistan and Afghanistan, ongoing political conflicts continue to challenge the eradication efforts.

Health officials also struggle with cases of vaccine-derived type 2 virus. Distinct from the three “wild” virus types, these cases are caused by a mutated version of the virus that was introduced by a phased-out polio type 2 oral vaccine.

As I’ve explained with Vox’s Brian Resnick, transmission of vaccine-derived polio works like this: A person swallows the oral vaccine, which contains a live, weakened virus. That virus can live in the gut for a while and then pass from person to person, developing mutations along the way. “When it circulates for a long time among too many poorly vaccinated or unvaccinated children, this is how it is allowed to mutate and become virulent again,” Michel Zaffran, director of the polio eradication program at the World Health Organization, told us.

In 2016, the Polio Global Eradication Initiative phased out this vaccine, switching to a safer oral vaccine. The outbreaks that are happening now were sparked before the switch.

So despite all the success beating back polio, some major challenges remain. But, as Richard Conniff writes in Scientific American, the polio program has a history of beating the odds:

India, where polio was paralyzing 500 to 1,000 children per day in the 1990s, eliminated the disease in 2014. The wrenching spectacle of child polio victims begging in that nation’s streets, with their twiglike legs folded beneath them, is now history. Nigeria, where antigovernment gunmen assassinated nine women polio vaccinators in 2013, has now gone three years without any evidence of wild poliovirus—and seven years without type 3—largely through the heroic persistence of community health workers.

The current goal for full polio eradication is 2023. “The achievement of polio eradication will be a milestone for global health,” said Tedros Adhanom Ghebreyesus, director-general of the World Health Organization and chair of the Global Polio Eradication Initiative, in a statement Thursday. “We remain fully committed to ensuring that all necessary resources are made available to eradicate all poliovirus strains.”