Media playback is unsupported on your device Media caption Anuradha Gupta, from India's Ministry of Health and Family Welfare, says they remain vigilant despite becoming free of the disease

Tackling polio has entered "emergency mode" according to the Global Polio Eradication Initiative after "explosive" outbreaks in countries previously free of the disease.

It has launched a plan to boost vaccination in Nigeria, Pakistan and Afghanistan, the only countries where the disease is still endemic.

Experts fear the disease could "come back with a vengeance".

The World Health Organization says polio is "at a tipping point".

There have been large outbreaks of the virus in Africa, Tajikistan and China has had its first cases for more than a decade.

'Relentless'

Bruce Aylward, head of the WHO's polio eradication campaign, said: "Over the last 24 months on three continents - in Europe, in Africa and in Asia - we have seen horrific explosive outbreaks of the disease that affected adults, and in some cases 50% of them died.

"What it reminded people is that, if eradication fails, we are going to see an huge and vicious upsurge of this disease with consequences that it is very difficult even to foresee right now."

He said the initiative was "now on an emergency footing" which would result in a "big shift" in the way the virus is tackled.

The strategy has been summarised as the "relentless pursuit of the unvaccinated child".

Battleground: Pakistan In the global fight against polio, Pakistan is the key battleground, with the highest number of cases in the world. Nearly 200 children were paralysed here in 2011 - the worst figures in 15 years. And the Pakistani strain of the virus has crossed borders - causing outbreaks in Afghanistan and China. The Pakistan government has already declared polio to be a national emergency. A small army of health workers - 88,000 - is targeting 33m children for vaccination. But officials admit as many as three quarters of a million children still have not been immunized. The government says immunization campaigns have been disrupted in recent years by a number of factors, including heavy flooding and military campaigns against the Taliban. There has been opposition too from some powerful clerics - and damage was done by the fake CIA vaccination campaign which helped to locate Osama Bin Laden in the Pakistani city of Abbotabad.

However, Dr Aylward also cautioned that there was a $950m shortfall in funding and admitted they had been forced into "cutting corners" with vaccination campaigns being stopped in some countries.

'Will'

India, once regarded as one of the most challenging countries, was declared free of the disease in February.

Kalyan Banerjee, the president of Rotary International, said: "We know polio can be eradicated, and our success in India proves it.

"It is now a question of political and societal will.

"Do we choose to deliver a polio-free world to future generations, or do we choose to allow 55 cases this year to turn into 200,000 children paralyzed for life, every single year?"

The Global Polio Eradication Initiative is a partnership between governments, the World Health Organization (WHO), Rotary International, the US Centers for Disease Control and Prevention and the United Nations Children's Fund.

Members of the WHO, meeting in Geneva, will vote this week on whether to declare polio eradication an "emergency for public health" in the three countries where it is still endemic.

The WHO estimates that failure to act could lead to as many as 200,000 paralyzed children a year worldwide within a decade.

The WHO originally set the year 2000 as its target for polio eradication. Dr Margaret Chan, director-general of the WHO, said the organisation was now working "in emergency mode".

The BBC's Imogen Foulkes in Geneva says the programme has claimed some remarkable successes, most notably India, which was declared polio-free in February.

She says the WHO hopes to shake donor countries out of their complacency and support one last effort at eradication. The WHO believes that with one last push, the disease could be eradicated globally, she says.

It is thought conflict and a lack of trust in vaccinations mean fewer children are being immunized.

Polio is a highly infectious disease caused by a virus. It invades the nervous system, and can cause total paralysis in a matter of hours.

One in 200 infections leads to irreversible paralysis, usually in the legs. Among those paralysed, 5% to 10% die when their breathing muscles become immobilized.

Unicef executive director Anthony Lake said: "All our efforts are at risk until all children are fully immunized against polio - and that means fully funding the global eradication effort and reaching the children we have not yet reached

"We have come so far in the battle against this crippling disease. We can now make history - or later be condemned by history for failing."