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.

More than 20,000 volunteers and health workers are making house calls to vaccinate children against polio in Afghanistan as part of a United Nations-backed campaign targeting nearly eight million children by the end of the year.

Nigeria and most other poor nations use an oral polio vaccine because it's cheaper, easier, and protects entire communities.

But it is made from a live polio virus — albeit weakened — which carries a small risk of causing polio for every million or so doses given. In even rarer instances, the virus in the vaccine can mutate into a deadlier version that ignites new outbreaks.