By Michelle Roberts
BBC News health reporter
Fifty years to the day since the discovery of a polio vaccine, the world is still not free of the disease.
Children are immunised before they are five
Six nations, some of the world's poorest, account for the thousand or so cases that occur each year.
Tremendous progress has been made since 1988 when mass immunisation was introduced at a time when 350,000 people caught the virus.
But far more work is ahead if it is to be eradicated completely, as smallpox has been.
The Global Polio Eradication Initiative set a target to wipe out polio infections by the end of this year.
Spokesman Oliver Rosenbauer said: "It's technically feasible, but there are a number of challenges to be overcome.
"We are facing a funding gap of over $75m. That has to be met by July for immunisation campaigns to go ahead later in the year.
"Then there is an outbreak of polio which is spreading across west Africa at the moment."
He said immunisation campaigns in some of the countries involved needed to be stepped up.
"In some areas, 20% of children are still being missed. That gives the polio virus enough breathing room to survive," he said.
India, Pakistan and Afghanistan are other hot spots.
Mr Rosenbauer said the fact that most countries in the developing, as well as the developed world, had eradicated polio was encouraging.
In the UK, the last reported case of polio was in 1982.
"But until it is gone from everywhere, we can't say it is unlikely to come back," he said.
Global Status in 2004
There were 1,263 cases in 2004 globally
Polio is still endemic in Nigeria, India, Pakistan, Niger, Afghanistan and Egypt
Source: Global Polio Eradication Initiative
Unless it is eradicated from the remaining six nations, the virus could spread and cause outbreaks in any country, he said.
Eradication would not be the end of the story.
It would still be important to maintain global surveillance to check for any new cases, said Mr Rosenbauer.
Oral immunisation would need to be phased out in synchronisation across the world.
It would also be necessary to ensure that any laboratory samples of the live virus were properly contained to prevent any accidental outbreaks, he said.
History of polio immunisation
An injectible polio vaccine was first developed in 1955 by Dr Jonas Salk from the US. This contained an inactivated or "killed" version of the virus.
An oral version was developed in 1961 by Dr Albert Sabin. Unlike the injectible form, the oral polio vaccine is a live attenuated, or weakened, version of the virus.
All children around the world are immunised before the age of five.
The oral polio vaccine provides good individual protection and better community protection, which is particularly important when the natural polio virus is circulating.
Former US president Roosevelt had polio
This is the version used in developing countries.
Countries that have eliminated polio tend to use the injectible, inactivated form.
This is because there is a small risk that the oral vaccine can itself cause disease because it contains live virus.
The protection offered by both vaccines is not 100%, so people who have been immunised can still become ill.
The polio virus invades the nervous system, and can cause total paralysis in a matter of hours.
It can strike at any age, but affects mainly children younger than three.
The virus enters the body through the mouth and multiplies in the intestine.
It is spread by poor sanitation, such as faeces infected drinking water.
Early symptoms are fever, fatigue, headache, vomiting, stiffness in the neck and pain in the limbs.
One in 200 infections leads to irreversible paralysis, usually in the legs.
World War II US president Franklin D Roosevelt fell ill with polio, which left him unable to walk without braces or a cane.
Rarely, some people will get fatal paralysis of their breathing muscles.