Nigeria marks three years on Wednesday since its last case of wild polio virus (WPV).

This date is significant because in order for the World Health Organization (WHO) to declare a country free of polio it has to go three years without a new case.

But it's not quite official yet that Nigeria is polio-free.

First the WHO has to work out whether the surveillance networks are robust enough so that they can safely say that there really have been no polio cases for three years.

That is not likely to happen until early next year, according to Chairman of Nigeria's polio committee, Dr Tunji Funsho.

Nevertheless, Nigeria has undoubtedly come a long way.

In 2012, the country had more than half of all polio cases worldwide, with 223 cases, the WHO says.

Dr Funsho puts this success down to, among other things, the reduction of fighting with Islamist militants Boko Haram.

The Boko Haram insurgency in north-east Nigeria had made some areas of Borno state hard to access.

That is reflected in where the most recent cases of polio have been and the last case of wild polio was detected on 21 August 2016 in Borno.