The World Health Organization certified India officially polio-free on Thursday after the country went three years with no new cases of the illness. The WHO’s Southeast Asia region, home to a quarter of the world’s entire population, is now also believed to be polio-free. Southeast Asia is the fourth of six WHO regions to officially eradicate the disease, meaning that 80 percent of the world’s population now lives in a polio-free area.

The possibility of a polio-free India was once unthinkable. As Maddy Fry at Time.com reports, over 50 percent of the world’s polio cases were concentrated in India, and the combination of the country's high population and areas of poor sanitation led to quick spread of the disease. Polio is usually spread through contaminated water or food, and children under five are particularly susceptible. 

In a ceremony in New Delhi held to declare the region’s eradication of polio, WHO official Dr. Poonam Khetrpal Singh called it “a day that we have dreamt about,” and said it’s “a day that all countries fought hard for, and a day when all stakeholders come together to celebrate the victory of mankind over a dreaded disease,” reports Manik Banerjee at the Associated Press. The Associated Press also found the last living person in India to contract polio, a 4-year-old girl named Rukhsar Khatoon. She contracted the polio in 2011 after her parents forgot to get her vaccinated.

Until polio is completely eradicated, all countries are at risk of reintroducing the disease, and there are still many cases in Nigeria, Pakistan and Afghanistan. In 2012, health care workers were killed in Pakistan while trying to deliver the vaccine after al Qaeda made threats to prevent a number vaccination drives, often accusing the doctors working for Western spy agencies.

In a statement, Singh said that successful polio eradication in India can pave the way for other health improvements in the country. "Now the polio programme has successfully reached them with polio drops in every round, there is no excuse not to go back with other critical health services, from how to have a safe birth, to where to get access to tuberculosis treatment and how to prevent HIV infection,” Singh said.

Member states in the WHO’s Southeast Asia Region are Bangladesh, Bhutan, Democratic People’s Republic of Korea, India, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, Thailand, and Timor-Leste.