Tank, Pakistan - Naheed Begum, clad from head to toe in a white garment, cuts a reluctant figure in this tiny alley in a congested residential area of the town of Tank, in northwestern Pakistan.

"I was afraid before I started this work. [I] do not feel safe out here, because there is a danger of being killed doing this work," Begum told Al Jazeera. "I do this because I'm desperate. My husband is sick, and there is no one else to earn in the house."

Naheed, 30, is normally a government worker, but on this bright afternoon in Tank, a small rural backwater that is one of the gateways to the tribal areas, she is going door to door on the front lines of one of Pakistan's biggest battles: the war against polio.

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Polio (poliomyelitis) is a highly infectious debilitating virus that targets the nervous system of children, causing partial or complete paralysis of their limbs. Since 1988, reported polio cases worldwide have declined by more than 99 percent, but the virus remains endemic in Pakistan, Afghanistan and Nigeria.

Last year, Pakistan had its worst year in more than a decade in terms of polio infections, with at least 306 cases reported, 85.2 percent of the 359 cases worldwide, according to the Global Polio Eradication Initiative.

The campaign faces many challenges, but perhaps the most pressing is the matter of continuing threats against polio vaccination teams issued by armed religious groups such as the Tehreek-e-Taliban Pakistan (TTP) and its allies.

Such groups allege the vaccine is part of a plot to sterilise or infect children, a belief that has been disseminated through many mosques and clerics in Pakistan.

Since July 2012, at least 66 people have been killed in attacks on polio vaccination teams across the country. The latest took place on March 18, when a health worker was killed and another wounded in an attack on a team in the Bajaur tribal area of Pakistan.

A day earlier, gunmen killed two more health workers and a police guard in a similar attack in the town of Mansehra.

'Alarming' situation

Tank, along with neighbouring South Waziristan, is one of the districts worst hit by polio infections, and has been identified by the World Health Organisation as posing serious challenges to national immunisation efforts.

Last year, Tank and South Waziristan accounted for 29 cases of polio - 9.5 percent of the national total - while this year the area has already registered its first case, according to local health authorities.

Tahir Javed, the district health officer for Tank, described the situation as "alarming".

"Tank is a small district, but the geographical and geopolitical situation is such that those areas that are adjacent to FATA [Federally Administered Tribal Areas], it's difficult for our teams to reach them," Javed told Al Jazeera, referring to South Waziristan and other tribal areas that together accounted for 53.2 percent of all polio cases in Pakistan.

"We have a vast shortage of staff [in Tank]. In the whole district, I have perhaps one medical officer, outside of the [main district] hospital," he said, explaining why polio immunisation efforts had suffered.

Polio is a water-borne virus, he explained, and is transmitted to children through the faecal-oral route. This is a particular challenge in a place such as Tank, where locals in the predominantly poor district often drink from the same surface or drainage water that they use for other household purposes.

"When that water flows in the drainage streams and goes into people's taps from there, then that isn't safe, is it? The water is coming from the rural areas, where people bathe in it, wash their clothes in it, and even use it as a toilet. So when our children drink this water [it is a danger for them]," he said.

Javed's point is underscored during a visit to one of the more densely populated parts of the city later in the day, where residents were seen collecting water from drains to boil and drink, and open sewage channels emptied themselves into alleys and streets.

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During one interview, one such drain emptied itself onto Naheed Begum and several other polio vaccinators, as well as this reporter.

Conspiracy theories

The threats to the polio eradication campaign are not limited to violence, a shortage of staff, and adverse environmental and hygiene conditions. Many parents, health officials told Al Jazeera, refuse to give their children the vaccine "on religious grounds".

"There are some rigid people who think that there is something harmful in the drops," said Dr Javed.

"They believe in conspiracy theories, which is a national pastime of ours, to see conspiracy theories. And indeed incidents such as Abbottabad have proved harmful to us," he added, referring to the CIA's use of a Pakistani doctor in running a fake vaccination campaign as part of efforts to track down al-Qaeda chief Osama bin Laden in 2011.

Field officers in polio vaccination campaigns have now begun to recruit the help of local Muslim clerics to dispel notions that the drops are prohibited by Islam, said Rehmat Gul, a member of the district polio monitoring team in Tank.

"There was a man who had an issue on religious grounds here," said Muhammad Dawood, a UNICEF staff member who was assisting the campaign in Tank. "I carry a book of fatwa's [religious edicts] with me, which I showed to him. And if he still doesn't agree, we talk to local clerics to take some time out to speak with [those who refuse the vaccine]."

The problem of refusals is a serious one, authorities say. During its last immunisation drive, Tank missed more than 15 percent of the 80,000 targeted children. The problem is not limited to poorer areas; during the same drive that Al Jazeera witnessed in mid-March, more than 16,400 families refused to have their children vaccinated.

About 610,000 children of the targeted 35.5 million were missed during the drive.

Making arrests

These refusals have come despite the fact the government has begun to arrest parents who refuse to administer the vaccine to their children. On March 1, authorities in Khyber-Pakhtunkhwa province arrested 471 people for refusing to give the vaccine to their children.

Nevertheless, authorities are cautiously optimistic about this year's campaign.

"At this time last year we had crossed 46 cases, this year we are at 20, so there has been a substantial and tremendous improvement, but still there are gaps. Still, much has been done, but more needs to do be done," Aziz Memon, the chairperson of Pakistan's National PolioPlus Committee, told Al Jazeera.

"There are areas which are still under the control of the militants where fighting is going on, and there are accessibility issues in those areas, plus there is a lapse on the part of the provincial governments."

Meanwhile, for the teams out in the field, the vaccinations and the dangers that come with it continue.

"I know that I can be targeted at any time. But I do this because it is a service to the nation, and these children are our own children," said Dawood.

"So we don't feel afraid, because it is not like we are doing a bad thing. We're doing a good thing."

Follow Asad Hashim on Twitter: @AsadHashim