One of the largest-ever campaigns to vaccinate kids against polio will take place in Africa this week in a global attempt to stamp out the disease, the World Health Organization announced.

The world is 99.9% free of polio right now, meaning that within the year polio could become the second-ever disease to be eradicated from humans (the first was smallpox, in 1980, following a similarly-massive push to vaccinate kids).

For this week’s enormous campaign, more than 190,000 vaccine workers will spread out across 13 African countries to reach 116 million children with polio immunizations. All kids under 5 years old will receive the vaccine.

The countries include Benin, Cameroon, Central African Republic, Chad, Cote d’Ivoire, Democratic Republic of Congo, Guinea, Liberia, Mali, Mauritania, Niger, Nigeria, and Sierra Leone.

Most of those countries have not had a recent case of polio, but last year, after being free of the disease for two years, Nigeria again saw an uptick in polio cases in areas held by the terror group Boko Haram.

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If neighboring areas have any vulnerabilities in their populations, the disease could spread, the WHO warned. The vaccine campaign this week is aimed at raising immunity in surrounding areas to prevent polio’s spread.

Just 20 years ago, every country in Africa was endemic to polio and Nelson Mandela launched the “Kick Polio Out of Africa” campaign. In the intervening years, the World Health Organization, governments of African countries, and NGOs have partnered to make dramatic progress getting rid of polio, which used to leave more than 75,000 children a year paralyzed in Africa alone.

The vaccine will be delivered to every house in cities, towns, and villages, of all of the countries, according tot eh WHO. The volunteers will travel by foot or bicycle up to 12 hours a day, carrying the vaccines in special ice-packed bags through 100 degree heat.

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"This extraordinary coordinated response is precisely what is needed to stop this polio outbreak," said Michael K McGovern, chair of Rotary’s International PolioPlus Committee.

Only three countries in the world still have cases of polio: Nigeria, Pakistan, and Afghanistan. With vaccine efforts ramping up in all three, advocates are hopeful that 2017 can be the year the the last case of polio is transmitted. It will still take years for the world to be declared fully polio-free, but this year could mark the beginning of freedom from a crippling, easily-prevented childhood disease.