If you stop your car at a traffic light in a West African city, you are as likely as not to have a polio victim tap on the window asking for charity

Polio can cause deformed limbs - and treatment and care of those affected strains the resources of many countries, but vaccination costs less than half a dollar per person.

The organisation says that since mass vaccinations began in the 1980s, the number of polio cases worldwide has been cut by 99%.

Wooden trolleys

But in parts of the world affected by conflict and poor sanitation, such as West Africa, the virus is still active.

If you stop your car at a traffic light in any west African city you are as likely as not to have a polio victim tap on the window asking for charity.

These people with deformed limbs, propelling themselves along on tiny wooden trolleys, are the most visible victims of the disease.

The oral polio vaccine saves millions of lives around the world

The WHO says previous mass campaigns against polio have been relatively successful, with the number of confirmed new cases in West Africa down from thousands a few years ago to just 20 so far this year.

But in countries without decent sanitation, the virus can live on, so this week health workers across the region are squeezing drops of vaccine into children's mouths in an effort to get rid of the disease completely.

The campaign includes sending volunteers to remote villages and tiny river islands.

In some cases governments have buried their political differences to allow vaccination teams to cross borders to access children in need.