The deaths this week of nine polio vaccine workers in Pakistan at the hands of gunmen indicate a threat not only to workers but also to the effort to eradicate the disease—locally and globally.

Earlier this year, the international push to eradicate the crippling—and sometimes deadly—childhood disease from its final holdouts (Pakistan, Nigeria and Afghanistan) was still in full force. Total worldwide cases had fallen to about 650 in 2011, and the incidence of the illness was down some 99 percent since the global launch of eradication efforts in 1988 by the World Health Assembly.

But in June, Pakistan's Taliban announced that it was prohibiting polio vaccinations in its tribal areas, which would prevent 160,000 children from receiving the preventive dose. The Taliban leaders said the move was to protest U.S.-led drone strikes.

The health care workers killed this week—presumably by Taliban militants—were shot as they made community rounds to vaccinate children during a three-day nationwide drive. The killings, which appear to have been coordinated, prompted the United Nations on Tuesday to remove its staff from the vaccination campaign and the Pakistani government to suspend the program in the Sindh and Khyber Pakhtunkhwa provinces, the BBC reported. Other areas, such as the city of Lahore, dispatched police escorts to accompany vaccination workers. On December 19, the suspension was instituted throughout all of Pakistan, although some efforts continued.

Polio is caused by a virus that destroys motor neuron cells, often crippling victims for life. It was effectively eradicated from the U.S. in 1979, but as long as it is circulating somewhere on the globe, it still posses a threat. The oral vaccine is not perfect. It contains a live version of the virus and can occasionally give polio to a child receiving the vaccination. In the U.S. and many other developed countries, the vaccination is given as a shot that contains inactivated virus, which cannot cause the disease. The injections, however, are more difficult to distribute and administer than the oral drops.

The worker attacks were shocking in themselves but they also "deprive Pakistan's most vulnerable populations—especially children—of basic life-saving health interventions," spokespeople for the World Health Organization noted in a prepared statement. Pakistan still has the most infections of any country. Skepticism and wariness of the vaccination campaign—with some inhabitants claiming health workers are attempting to sterilize children or that the workers are U.S. spies—have been flowing in the region for years but have surged since 2011, when a doctor was recruited by the CIA to run a mock hepatitis vaccination campaign to gather intelligence about Osama Bin Laden's whereabouts.

Most of the vaccine workers in Pakistan, including several of those killed, were young women, who were more easily able to gain access to households and speak with mothers. "The whole program is dependent on them," Imtiaz Ali Shah told The New York Times. Shah is a vaccine coordinator in Peshawar, an area near Taliban tribal land where at least two of the victims were killed that is also known to be a reservoir of polio infection. Some 225,000 workers and volunteers had been helping to administer the vaccine in Pakistan, and just one pair of workers can administer vaccine to 150 to 200 children in a single day, the Times reported. The health workers can also bring other public health messages to often rural, remote populations. "Every encounter a vaccinator has with a mother delivers other messages about breastfeeding, hand-washing or encouragement to take kids to health centers for other immunizations," Sarah Crowe, a spokesperson for UNICEF told the Times.

The tally of dead workers indeed seems to be greater than the number of children who will likely die from polio—which kills about 2 to 5 percent of those stricken—in Pakistan this year. But with a kibosh on vaccine rounds, other important public health messages will also go unspread.