The World Health Organization announced Friday that Nigeria has been removed from the list of polio endemic countries, a key step in the eradication of the disease.
The organization said Nigeria had gone more than a year without a recorded case of naturally occurring poliovirus. Once Nigeria has gone three years without reporting a case of the virus, the country can be declared polio-free.
The Global Polio Eradication Initiative, the public-private partnership that is leading the effort to eradicate polio, called the development a "historic achievement."
There are now only two countries, Pakistan and Afghanistan, in which active cases of the disease are found. In those nations, the transmission of the paralyzing virus has never been interrupted.
The president of the Bill and Melinda Gates Foundation, Chris Elias, hailed the achievement in Nigeria as a milestone but said the accomplishment "is also fragile."
Experts warn that continued success in the battle against polio depends on the continuation of vaccination campaigns and the close monitoring of suspected cases.
"This is a clear example of success under very difficult circumstances," said Tom Frieden, director of the U.S. Centers for Disease Control and Prevention. "It shows we can eradicate polio if proven strategies are fully implemented."
Nigeria has a long history of challenges surrounding its polio vaccination campaigns. In 2003, several northern states boycotted the polio vaccine over fears it could cause sterilization, leading to a resurgence of the virus in Nigeria and its spread to neighboring countries that had been declared polio-free.
In addition, nine vaccinators were killed in the northern state of Kano in 2013, and vaccinators have had difficulty reaching children in the northeastern state of Borno, which is at the heart of the Islamic insurgency by Boko Haram militants.
Despite the challenges, many government and community leaders in Nigeria have increased their support for the vaccination campaign, leading more families to seek out the vaccine for their children.
Polio, or poliomyelitis, is a highly infectious disease transmitted person to person, often through fecal matter and contaminated water. It mainly affects children, causing permanent paralysis and even death. Polio is easily prevented through vaccination, but there is no cure.