Rabail Mehar was at work when she got the news. Four women, polio vaccination workers like her, had been shot dead at three locations in Karachi within the space of 20 minutes. A fifth female worker had been killed in Peshawar.

“I started crying,” said Ms Mehar, who had only been in the position a month. “I decided I was going to quit.”

As it was, the 25-year-old did not resign on that dark, murderous day last December. Even though her mother and father wanted her to quit, that evening, with a “cool head”, she realised her role as part of the country’s campaign to end polio was more to her than just a job. It took three days to win over her parents.

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Across Pakistan there are thousands of women like Rabail Mehar. The battle to rid the country of polio is a team effort involving the central and provincial governments, private organisations and charities, Unicef and the World Health Organisation.

But on the frontline, where teams go door-to-door persuading often reluctant families to administer drops of vaccine to their children, it is women who are leading the fight. Frequently they risk their lives to do so.

There are lots of men out there too, but officials say women have generally shown themselves to be more effective at delivering the message that the vaccine can save a child from a debilitating disease. “Females feel more of a sense of responsibility,” said Dr Syed Saifur Rehman, a senior government official in Karachi. “They have better access. They are well dressed.”

Along with Afghanistan and Nigeria, Pakistan is one of three countries where polio remains endemic. In 2011 there were 198 confirmed cases, in 2012 there were 58 while this year there have been 24. The vast majority occur in the Federally Administered Tribal Areas and Balochistan.

But Karachi, with a shifting population of up to 25 million people and with large, displaced communities from Afghanistan and the tribal areas, is another of the battlegrounds. Two cases have been confirmed in 2013, the most recent on 8 August. It is 10.30am on another morning and the vaccination team has arrived at a shanty community opposite Karachi University, which is home to around 3,000 of the city’s most desperately poor. A mixture of Muslims and Hindus originally from the rural interior of Sindh province, the community here has no roads, drains or sanitation. “I want all the women to come out and bring your children,” says a man with a loud-hailer. “Be quick, we have limited time.”

Among the team is a young female supervisor wearing a blue scarf, pink nail polish and possessing a degree in international relations. The 27-year-old, who asked she be identified by the initial, S, said she was from a naval family and had two brothers and six sisters. After the attacks in December, S also came under pressure from her family to give up her post. They wanted her to take up teaching. She also refused. Now she supervises a team of “social mobilisers” who liaise with the migrant community and ensure the mothers don’t skip sessions.

At this location, most families agree to bring out their children even if they do not understand why the health workers are making such an effort – especially given they receive no other government services. “They come and do the vaccination. We don’t know about these things,” said Sarwan, 35, who sells fruit. A volunteer paints the nail of a child’s little finger with a purple marker to prove they were vaccinated against polio.

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Elsewhere things are not so simple. In some communities, especially in the remote and detached tribal areas, there is a commonly-held belief, spread by some clerics, that vaccinations are part of a Western plot to sterilise Muslims. Officials say this belief, and the number of cases of vaccine “refusals”, jumped in the aftermath of the US operation to kill Osama bin Laden when it emerged the CIA had recruited a Pakistani doctor, Shakil Afridi.

Dr Afridi had been tasked with establishing a fake vaccination campaign to obtain DNA samples from children living in Bin Laden’s Abbottabad compound. In 2012, Dr Afridi was jailed for 33 years after he was accused of treason. Last week a judge ordered a retrial.

Another aspect of the US’s so-called war on terror in Pakistan that has hindered the polio campaign is the CIA drone operation. Last year, the Taliban said polio workers could not operate in the mountainous area of Waziristan until the strikes were halted.

The decision meant 240,000 children under the age of five missed out on the vaccine. Sixteen polio cases have been detected in North and South Waziristan since the Taliban ban. Tests are being conducted on a further 42 suspected cases. “It’s real,” said a senior member of the Pakistan polio team, part of the Global Polio Eradication Initiative, who asked not to be identified. “If the drone strikes stopped, the polio campaign would improve.”

The suspicion aroused by Dr Afridi and the spectre of a US plot is underscored by the case of 32-year-old Usman, a labourer from the Bin Qasim Town neighbourhood. Even though Mr Usman suffered from polio as a child, he decided not to vaccinate his three-year-old son, Musharraf Khan, in the aftermath of the Dr Afridi affair.

“I saw that this job was being done by the CIA and its allies,” said Mr Usman, seated in an office of the Karachi health department.

Late last year, Musharraf Khan started complaining of weakness and fever. On 16 January he was positively identified as having polio. For several weeks it appeared he would be left disabled but intensive rehabilitation has seen his mobility return. Doctors say it must have been a mild case.

Most are not so lucky. Dr Abdul Samad, a WHO official with responsibility for the city’s northwestern Gadap Town area, pulled out his phone to play a video clip of one-year-old Sana Farishta, who was positively tested for polio on 8 August.

On the clip, the little girl’s father, Noor Mohammad, held her up but while she could put her weight on her right leg, the left flopped. Officials say the she had received nine dosages of the vaccine. They are unsure whether she is one of the rare cases in which the vaccine can be ineffective or if the cold-chain for the drug had been broken rendering it useless.

To counter public suspicion and a perception that taking the vaccine may somehow be unIslamic, the authorities have turned to influential members of the community. None have played a more important role than the Muslim clerics who have lent their support. Maulana Bilal Ahmed, a cleric in the Gulshan-e-Iqbal area in Karachi, said he had noticed the incidence of polio had reduced where children were given the vaccination. He now tells people to ensure their youngsters are vaccinated.

He also debates the issue with other clerics. “I always ask them for proof that the vaccine is harmful,” he said. “They cannot provide any, so that means I always win.” Even so, there are many within Pakistan who disagree. In the last 13 months there have been 24 attacks on vaccination teams. A total of 17 health workers and five police escorts killed and 14 injured.

While Ms Mehar’s parents were initially concerned about her safety, they now accept she is dedicated to the job. As it is, she is now the family’s sole earner, with the responsibility for five sisters and a brother. The eldest sibling, 18-year-old Naila, wants to go to medical college.

At the family’s home, Naila said that after the attacks in December, they wanted Ms Mehar to find a different job. “Because of her gift for this work, she carried out. She means everything to us,” she said.

As a full-time social mobiliser, Ms Mehar earns around £120 a month. Many of the women involved in the campaign are recruited on a daily basis and paid as little as £1.50.

Among the four polio workers killed in Karachi last December, at least two were paid on this ad hoc basis. Fahida and Madeha Shah, who were relatives, had set out close to their home in the Landhi Town district, when they were shot and killed, reportedly by gunmen on a motorbike. Gulnaz Sherazi, 30, who was Madeha Shah’s aunt and who is the campaign supervisor in the area, heard the gunshots. After some delay, she went to the scene.

When she got there she discovered Madeha lying in an ambulance. She assumed she was unconscious and reached out to touch her face only to find it was already cool. Ms Sherazi said Madeha was divorced and had a three-year-old child, who was now being raised by their grandmother.

Ms Sherazi said she had also received threats and after seeing her niece’s body lying in the ambulance she thought she would give up her job. She too decided to continue.

“We all die one day,” she said. “So why not die as a brave person?”

Eradication: a success story

Ever since the World Health Organisation set about trying to eradicate polio in 1988 with a huge vaccination drive the number of people – mainly children under the age of five – contracting the disease has fallen dramatically. Last year just 223 cases were reported, a 99 per cent drop from the 350,000 cases in 2008.

Similarly, the number of countries where the virus is still endemic has fallen from 125 in 2008 to just three now – Pakistan, Nigeria and Afghanistan.

This is the smallest geographical area for the disease in history. But WHO warns that unless it is wiped out in these last bastions, all its good work will be undone, and infection rates could rise to around 200,000 across the world in the next decade.

Polio is spread by a virus which infects the central nervous system after entering the body through the mouth. There is no cure and it can cause total paralysis in hours. One in 200 of those who contract polio will become irreversibly paralysed, usually in the legs. Of those, 5 to 10 per cent will die when their breathing muscles become paralysed.

Unlike other viruses, however, polio can be wiped out. Only three strains exist and it cannot survive outside the human body.

Andrew Johnson