By Karen Allen BBC Health correspondent

India is embarking on one of the most ambitious health programmes ever - immunising children against polio, which is a totally preventable disease.

Poverty, poor sanitation and a dense population make this a fertile ground for infection

In six days it hopes to immunise about 160 million people. India is one of the last outposts where polio has a hold.

Polio which used to paralyse 350,000 people a year - a third of them in India - is on the verge of becoming extinct.

International agencies had hoped to complete the task by 2005, but with Uttar Pradesh in India registering a five fold increase in cases over the past two years, hopes of reaching that milestone look slim.

Slipping through the net

The northern densely populated state accounts for 68% of all infections in the world so the fate of the global eradication programme hinges on what can be achieved here in the coming months.

For less than a dollar a dose, oral polio vaccine offers simple protection.

But the World Health Organisation (WHO) stresses that four doses are needed to ensure protection, before a child turns one year old..

The problem in Uttar Pradesh is that children are slipping through the net.

Poverty, poor sanitation and a dense population make this a fertile ground for infection.

And though other parts of India have now become disease free - Uttar Pradesh has bucked the trend.

Fifteen hundred new cases were diagnosed last year.

It may not sound many but so long as the virus persists, gains made in neighbouring Bangladesh and Sri Lanka (both polio free) could unravel.

Mistrust

No-one knows precisely why polio persists in a state that has produced more prime ministers than any other, but mistrust of authority has certainly played a part.

The Muslim minority account for 70% of new infections here.

Many have rejected the polio vaccine, wary of government motives in this Hindu-dominated state.

The door-to-door campaign has echoes of failed health programmes of the past - in particular the sterilisation drive of the 1970s that met with global disapproval, and rumours circulate which claim that polio vaccine makes you infertile.

Red tape lifted

Dr Sobhan Sarkar, India's minister in charge of the vaccination programme, admits that mistakes in the past and complacency may account for the recent rise in cases in Uttar Pradesh.

But he insists the situation has now changed and polio has adopted a higher political profile.

Administrative bottlenecks that hampered the programme before, he claims, have been eased.

This may not be a disease with the impact or scale of HIV/Aids, but eliminating polio from the world could mean the need for routine polio immunisation everywhere could cease.

Only seven countries in the world still have the wild polio virus circulating amongst its population, but India, followed by Nigeria, still have a persistent problem.

With hopes that global eradication could be reached by 2005 now dashed, the world is turning its focus on India to reverse its misfortunes.