Deep in the slums of Pakistan's biggest city there is a small boy who is a warning to us all.
His name is Mustafa, he is 15-months-old, and we watch him crawl across the dirt floor of his home. It is only when his mother tries to stand him up that his difficulties becomes obvious.
A leg that is visibly weak and a foot that is twisted and hangs uselessly at an odd angle.
He is suffering from a disease that could, perhaps should, have been conquered long before he was born.
Watch John Ray's report from Karachi:
Footage credit: Evan Williams Productions
Instead, while most of the world has consigned polio to history, Mustafa's world will be confined by its crippling effects. Polio was once a common curse of Pakistan's poor.
Now, on these same back streets, a desperate battle is underway to prevent its return. At times the battle can seem like a real shooting war.
Only under armed guard do the teams who administer the vaccine - that promises immunity - dare reach out to the impoverished communities most at risk.
We accompanied a team in Karachi; where in the past two years, seven health workers have died out of a total of 26 nationwide - gunned down by Islamic militants.
Anita Jawed, a 32-year-old volunteer, is among those who gave her life in the drive to immunise children.
"She died trying to help her country," her tearful mother told us. "She is a martyr." It sounds like scant consolation.
Dr Akbari Khanoom is a leader of the polio campaign in Karachi.
She tells us people are suspicious of the vaccine. Many believe it is part of a secret American plot to sterilise their children.
It doesn’t help that the CIA used a fake health programme to gather DNA samples in the hunt for Osama Bin Laden.
In Karachi, senior religious leaders have defied the Taliban to issue a fatwa that tells parents to bring their children for vaccination.
Slowly the message is getting through, but if things are bad in Karachi, in Pakistan's tribal areas to the north, dominated by the Taliban and other hardliners, the situation is critical.
There, a quarter of a million children have missed out on vaccination.
Pakistan's Prime Minister has created a special unit within his own office to monitor and tackle the disease.
Dr Altaf Bosan of the unit told me he is confident polio will be controlled but for the moment, within the tribal areas, the dangers of an epidemic are real.
Those fears prompted the US this week to promise publicly to never again use a fake vaccination programme as cover for intelligence gathering.
It is an assurance that will be lost on many - and in any case comes too late for the likes of Mustafa.