LAHORE – They gathered in a small room in one of this city’s worst slums, a dozen mothers sitting cross-legged with toddlers and newborns on their laps, listening to advice about polio prevention.

“Keep your children from playing in garbage cans and sewer drains,” said Saddaf Malik, a brightly dressed young woman from UNICEF.

Simple enough, but then came the questions, spiked with suspicion and indicative of why Pakistan remains one of three countries in the world where the paralyzing disease still thrives despite constant campaigns in recent years to defeat it.

Why, some mothers wondered, were the vaccination teams coming back once a month, instead of every three months like they used to? Were the repeated doses of the red drops meant to induce sterility in Muslims?

The polio fighters looked crestfallen. They thought this dangerous myth was dead in Lahore – a sprawling city with an estimated 1.5 million young children that logged just one polio case a year ago and none since.

Troubling, too, was the confirmation of the polio virus in 16 of 28 sewage samples taken so far this year in Lahore, a marked increase over 2011. And three successive positive samples – in July, August and September – have raised worries about the virus’ “silent circulation,” as World Health Organization officials call it.

“We take it very seriously if there is even one positive sample,” said Ni’ma Abid, a WHO senior medical officer. “It means you have polio in the community.”