Gunmen in the Pakistani city of Quetta killed three female polio vaccination workers and their driver Wednesday, the latest bout of violence to target immunization efforts in one of the last countries in the world where the disease has not been eradicated.

Ayesha Raza, spokeswoman for Pakistan's polio eradication program, said the attack, which left three others injured, occurred when two armed men on motorcycles opened fire on the workers' van as they traveled to meet a police escort.

Human Rights Watch estimates at least 35 health workers have been killed this year during vaccination efforts. On Monday, militants reportedly from the Pakistani Taliban attacked a vaccination team in Khyber Pakhtunkhwa province in the country's northwest, leaving one worker injured.

Health officials have counted 265 cases of polio in Pakistan this year, accounting for more than 80 percent of cases worldwide in 2014. The polio toll in Pakistan is higher than at any point in the last 15 years. At this time last year, the country suffered only 64 cases. Polio is also endemic in Afghanistan and Nigeria.

Pakistani military campaigns in the restive North Waziristan region abutting Afghanistan have displaced thousands of people, including many unvaccinated children. According to the Global Polio Eradication Initiative, which tracks the disease, "North Waziristan is the district with the largest number of children being paralyzed by polio virus in the world."

The attack Wednesday occurred in Baluchistan, where militants are waging an ongoing battle with the Pakistani government. The United Nations reported 14 new cases of polio in Pakistan in the past week, including three in Baluchistan, five in Khyber Pakhtunkhwa province, and five more in the Federally Administered Tribal Areas, an area that includes North Waziristan.

Militants have long associated vaccination efforts with Western intervention and spying in Pakistan. Those suspicions proved legitimate when the CIA had a local doctor deploy a fake vaccination program in Abbottabad in an attempt to gain information on Osama bin Laden, who was later killed in a nighttime American raid. American drone attacks have only heightened the stakes for vaccination workers.

"There has also been opposition from religious clerics in Pakistan, who peddle the most fantastic conspiracy theories — the most popular one being that the polio vaccine is a Western conspiracy to make Muslim men infertile," Saroop Ijaz, a lawyer working with Human Rights Watch in Pakistan, told VICE News.

"But the overwhelming majority of Pakistanis — that includes religious people and religious leaders — are not averse to polio vaccination," Ijaz added. "But there are enough people who are willing to be violent to create this horrific situation."

Ijaz said the vaccination workers — mostly poor women — enjoy little support or protection from the government.

"These women are already disenfranchised and make for easy targets," Ijaz said. "They are paid less than the minimum wage, which ranges from 250-500 rupees a month, or $5 dollars."

According to UNICEF, which helps coordinate polio eradication efforts in Pakistan, the influx of refugees from North Waziristan increases the risk of further transmission, but also allows workers to vaccinate more Pakistanis. Vaccination teams have set up mobile clinics at points along routes taken by displaced people. UNICEF reports that more than 1 million people have been vaccinated at these sites, or in host communities in the past several months, including 850,000 children under 10 years old.

"Yes, you are seeing a rise in polio cases but we are also optimistic that with good management and oversight we can reach children we haven't reached before," Shelley Thakral, senior communications manager for UNICEF's polio program in Pakistan told VICE News.

Most patients carrying polio don't realize they have it, but remain infectious. The virus can be transmitted through saliva and feces. In some cases, infections can lead to sudden death or paralysis.

Walter Orenstein, associate director of Emory University's Vaccine Center, told VICE News the conditions in Pakistan are a worst-case scenario for transmission of a disease that should no longer exist in humans.

"In areas where there is poor hygiene and sanitation and crowding, the place can be contaminated by the virus," Orenstein said. "Children in particular are the least hygienic."

Orenstein noted that the oral vaccine used by workers in Pakistan is important because it induces immunity in a person's intestines, not just their blood. Those who receive traditional injected vaccines may be protected from the virus, but can still transmit it through their feces.

"The big thing is to have people already immune at the time someone with the virus comes in contact with them," Orenstein said.