(A female polio worker gives polio vaccine drops to a child in Lahore December 20, 2012. REUTERS/Mohsin Raza)

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.