By Jill McGivering
BBC News, Islamabad
Vaccination teams have been unable to reach many troubled areas
The continuing conflict in Pakistan has left more than a million children at risk of contracting polio.
Vaccination programmes earlier this year were compromised by opposition from militants and by violence, which has made many areas inaccessible.
Pakistan is one of four countries which still see annual outbreaks of polio.
The first few months of the year are crucial for vaccination teams who try to reach children before the polio virus starts to circulate.
This year, their campaigns have been badly disrupted.
In some places, Taliban extremists have intervened and stopped programmes, calling them un-Islamic.
The conflict will create a cluster of children, a cohort, who are susceptible to the virus
Dr Khalif Bile Mohamud,
World Health Organisation
Some militants have opposed the vaccine, describing it as dangerous and part of a Western plot to harm Muslim children.
But perhaps the biggest obstacle has been the fighting.
Since the start of the military offensive against the insurgents, many areas, including much of Swat and parts of the tribal areas, have become inaccessible to health teams.
Dr Khalif Bile Mohamud, the World Health Organisation representative in Pakistan, says more than a million children, trapped inside the conflict zone, have been missed.
"Pakistan was the country closest to interrupting the virus. Now we have a challenge - the conflict will create a cluster of children, a cohort, who are susceptible to the virus," he said.
Health teams are trying to vaccinate as many children as possible who have emerged from the conflict zone because their families fled the violence.
They have already treated about half a million. The aim is to create a firewall of immunity - so, if there are outbreaks they can be contained.
So far this year 20 cases have been confirmed - seven of them from north-west Pakistan.
But this is just the start of the polio season. Most cases are usually reported in late summer and autumn.
And accurate information may be hard to come by. The surveillance system is also being compromised by the insecurity.