Polio is once again spreading in Pakistan, where officials say more than 1 million children missed their vaccination doses last month, underscoring the challenges they face in eradicating one of the world’s most intractable diseases.

Polio cases are rising ahead of a new vaccination campaign in Pakistan, where violence targeting health workers and the police protecting them has hampered years of efforts toward making the country polio-free.

Every paralytic polio case means there are hundreds of children who are silently affected by poliovirus and are potentially carrying and spreading it throughout their communiti