Watch part two
Filmmaker: Joanna Head
Polio is a devastating disease that still exists in some parts of the world, despite an unprecedented global health campaign to get rid of it.
Children are particularly at risk and Indian-born Gautam Lewis was just one among many millions to contract the virus.
Raised in a Calcutta orphanage, his life might have remained one of grinding poverty - had he not been adopted at the age of seven and taken to live in privilege and comfort in Britain.
Twenty three years on, Gautam leaves his London home and returns to the country of his birth.
India is one of just four countries in the world where polio remains endemic and Gautam travels back to take part in National Immunisation Day when, it is hoped, more than 75 million children will be vaccinated.
The battle to wipe polio from the planet is the largest public health initiative the world has ever known - poised on a knife-edge between success and failure.
If it fails, more than 10 million children will be paralysed in the next 40 years.
Cases of polio have declined from 1,000 a day to just 2,000 a year. Endemic polio has been erased in all but four countries - India, Pakistan, Afghanistan and Nigeria.
Despite the utter chaos in Somalia, polio has been eradicated; in Afghanistan, mediators negotiate ceasefires with the Taliban to get children vaccinated.
But India is the key - it still has the highest number of polio cases in the world.
So far it has cost $5 billion and involved 20 million vaccinators across the world. But all this money and effort could be wasted if the disease is not now erased forever.
As long as polio exists in one country, every country must keep vaccinating. The Gates Foundation has just promised $100 million of the $200 million needed to fund the final push towards global eradication.
"If we don't eradicate it now we never will - all the energy and commitment, all the billions of dollars will have been wasted. The virus will have won. It is an unbearable thought," said Jonathan Veith from WHO.
It is an international story of epic proportions that highlights the problems - and achievements - of an extraordinary global collaboration.
Passport from Polio can be seen from Thursday, April 8, 2010 at the following times GMT: Tuesday: 0830, 1900; Wednesday: 0330, 1400, 2330.