The biggest ever public health drive is under way to immunise 300 million children in Africa and Asia against the potentially crippling disease of polio.
Nigeria has the highest rates of polio in the world
Tens of thousands of mobile teams run by the World Health Organisation are working at locations including Egypt, Afghanistan, Pakistan and India.
The aim is to reach 80 million children under five in sub-Saharan Africa alone.
This year, 90% of polio cases have been in Africa, with some areas seeing near epidemic rates of transmission.
The WHO said earlier this year that it aimed to eradicate polio by March 2005.
But its Global Polio Eradication Initiative suffered a major setback last year, when vaccinations stopped in northern Nigeria because of safety fears.
Two campaigns have already been completed here, so the region could catch up on the immunisations missed over the last year.
Other immunisation programmes are underway in India and Pakistan - two of the Asian countries, along with Afghanistan, where the disease is endemic.
In just one year, however, the virus has spread to 12 previously polio-free countries.
'Largest' epidemic risk
Armed conflict in Sudan's Darfur region has played a part in spreading the disease, according to the WHO.
It said Sudan had been polio-free since April 2001, but cross-border movement between Chad and Sudan had re-introduced
Fighting in the area had stalled immunisation campaigns, it said.
Now 23 nations in west and central Africa are taking part in the biggest ever synchronised vaccination project in the region.
As it is now polio high season and infections are much more likely, over a million polio vaccinators are working in Africa - and even more are visiting homes in parts of Asia.
"For the next few months Africa really faces the risk of the largest epidemic of polio in the recent history," says Rimah Salah of UN children's fund, Unicef
"It is threatening thousands of children and jeopardising our common investment in a polio-free world."
However, the campaign in parts of eastern Democratic Republic of Congo was suspended before it had even begun because of insecurity.
Because so few countries now immunise children against polio, the stakes are very high.
If the virus spreads further, many people simply will not be protected.
So far $3bn has been spent on polio eradication since 1988, but more is needed to complete this campaign.
Two more mass vaccinations are also planned - one next month and the other in early 2005 - so even more cash must be found.
But by then it will be known if transmission has stopped and if the world's biggest ever public health campaign has been a success.