ROBERT SIEGEL, HOST:
There's a race on. The way to win is to eradicate a human disease. That's only been done once before - smallpox. This year, two diseases got tantalizingly close, but unexpected roadblocks have popped up. NPR's Michaeleen Doucleff reports.
MICHAELEEN DOUCLEFF, BYLINE: Back in 2012, everyone in global health was talking about polio - the end of polio.
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MELISSA BLOCK, BYLINE: Now the decades long fight against polio has reached what public health officials hope is a final showdown.
DOUCLEFF: That's NPR's Melissa Block. The number of polio cases around the world had plummeted. Eradication was in reach, but a few years later, polio made a comeback.
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BLOCK: Officials at the WHO are concerned about polio outbreaks in Pakistan, Syria and Cameroon.
DOUCLEFF: Eradication was slipping away just as another disease neared the finish line - Guinea worm.
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UNIDENTIFIED MAN: For thousands of years, the Guinea worm parasite has caused disabling misery.
DOUCLEFF: That's a promotional video from the Carter Center, which has led a 30-year battle against the painful disease.
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UNIDENTIFIED MAN: The adult worm, up to 3 feet long, emerges from the body through agonizing skin blisters that incapacitate and cripple.
DOUCLEFF: In 2013, former President Jimmy Carter had some good news to share.
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JIMMY CARTER: We hope and expect that within a year or two, there will be normal more Guinea worm anywhere.
DOUCLEFF: And by 2017, polio eradication was back on track. Now, it looks like both diseases are super close to eradication. Dr. Michel Zaffran at the World Health Organization says there were just 20 polio cases this year. That's the lowest number ever recorded in human history.
MICHEL ZAFFRAN: We started with 350,000 cases per year in 1988. So, you know, it's a reduction by 99.99 percent. It's an extraordinary achievement.
DOUCLEFF: And for Guinea worm, there have been just 26 cases. Dr. Sharon Roy at the Centers for Disease Control and Prevention says one of the worst-hit countries, South Sudan, seems to have finally wiped out the worm.
SHARON ROY: It is a huge deal, a huge, huge story. So we have our fingers crossed, and we are holding our breath.
DOUCLEFF: And now, here's the bad news. Let's start with Guinea worm.
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DOUCLEFF: They have a big, furry problem.
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DOUCLEFF: Yep - dogs. A few years ago, Guinea worm started appearing in lots of dogs in Chad. There have been more than 750 cases in dogs this year, dogs that are allowed to roam around and spread Guinea worm.
ROBBIE MCDONALD: They have a pretty free-ranging existence and are able to spend their nights and days as they wish.
DOUCLEFF: That's Robbie McDonald, an ecologist at the University of Exeter. He says a problem is scientists don't know how the dogs are getting Guinea worm. They've even done a forensics analysis on the dogs' whiskers to see what they're eating but it was inconclusive. Until they know how dogs are being exposed, they can't stop the outbreak in people. Polio's problem is more serious because the tool being used to fight the outbreak is the problem. There's a defect in the polio vaccine. You see, the vaccine is a weakened form of the virus, which WHO's Dr. Zaffran says can regain power.
ZAFFRAN: In very special circumstances, the vaccine will gradually mutate. And in rare cases, it regains its ability to become virulent.
DOUCLEFF: And cause paralysis in unimmunized kids. This is exactly what happened this year in the Democratic Republic of Congo and Syria. Those countries recorded 84 cases of vaccine-derived polio.
ZAFFRAN: We are surprised by the magnitude of the Syrian outbreak which is very, very significant. We've never had such a large outbreak of this type.
DOUCLEFF: In fact, for the first time in history, the virus from the vaccine has caused more cases of polio than the regular wild polio. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.
