The global fight to eradicate polio has set its sights on Afghanistan and Pakistan, the World Health Organisation (WHO) said on Tuesday as Africa marked a year without any new cases of the crippling disease for the first time.

Nigeria is set to be removed from the list of polio-endemic countries, leaving only Afghanistan and Pakistan, and raising hopes that it will soon become the second human infectious disease after smallpox to be eradicated.

Of the 34 new polio cases reported so far in 2015 - 28 of them have been in Pakistan, with the rest in Afghanistan.

“Pakistan and Afghanistan need to finish the job as quickly as possible. They are the source of the fire and you don't want it spreading again as it has done in the past,” said Oliver Rosenbauer from the WHO's Global Polio Eradication Initiative.

Until the development of a vaccine in the 1950s, the polio virus invaded the nervous systems of thousands of people annually, rich and poor alike, causing irreversible paralysis within hours.

It can spread rapidly, particularly among children and especially in unsanitary conditions in war-torn regions, refugee camps and areas where healthcare is limited.

As in Nigeria and Somalia, the last African nation to go a year without a case of polio, running vaccination campaigns in Afghanistan and Pakistan have proved challenging due to political instability and internal conflict, Rosenbauer said.

“You also have daunting challenges like huge population movement and slum areas where it is difficult to know how many children there are,” he said.

But Pakistan had made great progress in the past year as it ran its first eradication program, he added, and WHO had seen significantly more children vaccinated in tribal areas where the government has limited control and access.

WHO estimates global savings of $50 billion over the next 20 years if polio is eradicated worldwide, but say that failure to do could result in up to 200,000 new cases every year, within 10 years.

Somalia, on Tuesday, marked a year without recording a polio infection, six weeks after Nigeria reached the same milestone.

But the continent must go another two years without a new infection to be declared polio free, and WHO urged both countries to avoid complacency and redouble their efforts to keep the disease at bay.