In the fight against polio, the most vulnerable are often the hardest to help.
And it is not just the children of Pakistan's tribal areas, long a virtual no-go area for the vaccination teams.
Hidden under a flyover, we found the poorest of Karachi’s poor, living amid mounts of rubbish and human waste.
It is the ideal breeding ground for a disease that in the summer heat spreads rapidly from contaminated hand to mouth.
Their make shift shelters are next to a slow moving river that is also a sewer.
Recent tests have proved the waters also harbour the polio virus.
Those whose future is already so limited by poverty are easy prey to a disease from the past.
There we meet Fatima, a mother of eight children. She says they haven't seen a vaccination team for many months.
"We are poor, no one care about us,'' she says.
For generations polio ran its ruinous course unchecked.
Although new cases are still measured by the score (the 67th this year was recorded today) rather than the thousands of twenty years ago, they are on the rise again.
And only those forced to live with it know the misery that brings.
Abrar Khan contracted the disease at the age of three.
"My father took me to five doctors. The last told him I had polio,'' he told me.
These days he works with UNICEF, warning parents to get their children immunised or risk gambling with their future.
"I tell them life for a disabled person here is very hard. You are a burden,'' he says.
For only the second time since 2007, the World Health Organisation has declared a public health emergency.
At airports, travelers are required to show proof of vaccination or take polio drops.
The Pakistan strain of the virus has already been found as far afield as Cairo and Tel Aviv.
The 25 year campaign to eradicate polio across the globe is suddenly in reverse.
"It is said that the virus doesn't travel by itself, it travels with people,'' says Dr Zubair Mufti, WHO's Senior Polio Surveillance officer in Pakistan.
"So really, the thing can spread and really make havoc.''
The coming weeks and months when the virus is at its most potent might be make or break for the campaign.
Until every one of Pakistan's children is safe - nowhere in the world can be counted truly free of this crippling scourge.