By Mehreen Zahra-Malik

ISLAMABAD, Pakistan (Reuters) - Pakistani health worker Bushra Bibi spent eight years trekking to remote villages, carefully dripping polio vaccine into toddlers' pursed mouths to protect them from the crippling disease.

Now the 35-year-old mother is too scared to go to work after masked men on motorbikes gunned down nine of her fellow health workers in a string of attacks this week.

"I have seen so much pain in the eyes of mothers whose children have been infected. So I have never seen this as just a job. It is my passion," she said. "But I also have a family to look after ... Things have never been this bad."

After the deaths, the United Nations put its workers on lockdown. Immunizations by the Pakistani government continued in parts of the country. But the violence raised fresh questions over stability in the South Asian nation.

Pakistan's Taliban insurgency, convinced that the anti-polio drive is just another Western plot against Muslims, has long threatened action against anyone taking part in it.

The militant group's hostility deepened after it emerged that the CIA - with the help of a Pakistani doctor - had used a vaccination campaign to spy on Osama bin Laden's compound before he was killed by U.S. special forces in a Pakistan town last year.

Critics say the attacks on the health workers are a prime example of the government's failure to formulate a decisive policy on tackling militancy, despite pressure from key ally the United States, the source of billions of dollars in aid.

For years, authorities were aware that Taliban commanders had broadcast claims that the vaccination drive was actually a plot to sterilize Muslims.

That may seem absurd to the West, but in Pakistan such assertions are plausible to some. Years of secrecy during military dictatorships, frequent political upheaval during civilian rule and a poor public education system mean conspiracy theories run wild.

"Ever since they began to give these polio drops, children are reaching maturity a lot earlier, especially girls. Now 12 to 13-year-old girls are becoming women. This causes indecency in society," said 45-year-old Mir Alam Khan, a carpet seller in the northern town of Dera Ismail Khan.

The father of four didn't allow any of his children to receive vaccinations.

"Why doesn't the United States give free cures for other illnesses? Why only polio? There has to be an agenda," he said.

While health workers risk attacks by militants, growing suspicions from ordinary Pakistanis are lowering their morale. Fatima, a health worker in the northwestern city of Peshawar, said that reaction to news of the CIA polio campaign was so severe that many of her colleagues quit.

"People's attitudes have changed. You will not believe how even the most educated and well-to-do people will turn us away, calling us U.S. spies and un-Islamic," said the 25-year-old who did not give her last name for fear of reprisals.

"Boys call us names, they say we are 'indecent women'."

Pakistan's government has tried to shatter the myths that can undermine even the best-intentioned health projects by turning to moderate clerics and urging them to issue religious rulings supporting the anti-polio efforts.

Tahir Ashrafi, head of the All Pakistan Ulema Council, said the alliance of clerics had done its part, and it was up to the government to come to the rescue of aid workers.

"Clerics can only give fatwas and will continue to come together and condemn such acts," he said. "What good are fatwas if the government doesn't provide security?"

RISK OF POLIO RETURNING

That may be a tall order in Pakistan, where critics allege government officials are too busy lining their pockets or locked in power struggles to protect its citizens, even children vulnerable to diseases that can cripple or disfigure them.

Pakistani leaders deny such accusations.

Politicians also have a questionable track record when it comes to dealing with all the other troubles afflicting nuclear-armed Pakistan.

The villages where health workers once spent time tending to children often lack basic services, clinics, clean water and jobs. Industries that could strengthen the fragile economy are hobbled by chronic power cuts.

Deepening frustrations with those issues often encourage Pakistanis to give up on the state and join the Taliban. Continued...