Last week, Nigeria met an important milestone: An entire year without a reported case of polio. If the WHO confirms the absence of the virus in samples taken from people in previously affected areas, Nigeria will no longer be on the list of countries where the disease is endemic. Another two years will have to pass without additional cases before the WHO can certify Nigeria — and possibly the entire continent of Africa — as polio-free.

India was in a similar position a few years ago: In 2012, it was removed from the list of polio-endemic countries, and the WHO declared it polio-free in 2014. In 2015, only Pakistan and Afghanistan had any reported polio cases.

TIME’s Jeffrey Kluger reports on how Nigeria came this far:

The victory in Nigeria did not come easy—and it almost didn’t happen at all. For more than a generation, it has been Rotary that has led the drive to eradicate polio, administering vaccinations to 2.5 billion children in 122 countries at a cost of $1.4 billion. With the help of UNICEF, the World Health Organization (WHO), the U.S. Centers for Disease Control, the Bill and Melinda Gates Foundation and other groups, the effort paid off comparatively fast. As long ago as 2003, the virus had been chased out of all but six countries and the global caseload was down to just 732. There was talk of eradication by as early as 2005.

But Nigeria scuttled those plans. In the summer of 2003, Muslim clerics in the country’s northern regions halted all vaccinations, spreading the fiction that the vaccines contained HIV and were designed to sterilize Muslim girls. Quickly, the poliovirus did what all viruses do when they’re given that kind of running room: it spread, and fast. By 2005, cases consistent with the Nigeria strain were appearing in a 16-nation band that stretched as far away as Indonesia, before the outbreak could finally be contained.

… While the current victory in Nigeria was a huge milestone, things remained dicey right to the end—again due to politics—when Boko Haram fighters killed nine polio workers and abducted three others earlier this year. But the vaccine program was already too far along for the attacks to reverse things, and as the July 24 anniversary arrived, victory was at last declared—albeit tentatively.

Nigeria isn’t the only country where vaccination workers have been killed. The deaths of nine polio vaccinators in Pakistan in 2012 followed a 2011 report that the CIA had used a fictional vaccination campaign to gather intelligence on Osama Bin Laden. Since then, a total of 65 health workers have been killed in Pakistan.

Vaccination workers are risking their lives for a worthwile goal: eradication of a disease that once paralyzed hundreds of thousands of children each year. The world is drawing closer to this goal, but the BBC reminds us: “As long as a single child remains infected with polio, unvaccinated children in all countries are at risk.”